Subroutines

Like any good programming langauge Perl allows the user to define their own functions, called subroutines. They may be placed anywhere in your program but it's probably best to put them all at the beginning or all at the end. A subroutine has the form 

sub mysubroutine
{
	print "Not a very interesting routine\n";
	print "This does the same thing every time\n";
}
regardless of any parameters that we may want to pass to it. All of the following will work to call this subroutine. Notice that a subroutine is called with an & character in front of the name: 
&mysubroutine;		# Call the subroutine
&mysubroutine($_);	# Call it with a parameter
&mysubroutine(1+2, $_);	# Call it with two parameters

Parameters

In the above case the parameters are acceptable but ignored. When the subroutine is called any parameters are passed as a list in the special @_ list array variable. This variable has absolutely nothing to do with the $_ scalar variable. The following subroutine merely prints out the list that it was called with. It is followed by a couple of examples of its use. 
sub printargs
{
	print "@_\n";
}

&printargs("perly", "king");	# Example prints "perly king"
&printargs("frog", "and", "toad"); # Prints "frog and toad"
Just like any other list array the individual elements of @_ can be accessed with the square bracket notation: 
sub printfirsttwo
{
	print "Your first argument was $_[0]\n";
	print "and $_[1] was your second\n";
}
Again it should be stressed that the indexed scalars $_[0] and $_[1] and so on have nothing to with the scalar $_ which can also be used without fear of a clash.

Returning values

Result of a subroutine is always the last thing evaluated. This subroutine returns the maximum of two input parameters. An example of its use follows. 
sub maximum
{
	if ($_[0] > $_[1])
	{
		$_[0];
	}
	else
	{
		$_[1];
	}
}

$biggest = &maximum(37, 24);	# Now $biggest is 37
The &printfirsttwo subroutine above also returns a value, in this case 1. This is because the last thing that subroutine did was a print statement and the result of a successful print statement is always 1.

Local variables

The @_ variable is local to the current subroutine, and so of course are $_[0], $_[1], $_[2], and so on. Other variables can be made local too, and this is useful if we want to start altering the input parameters. The following subroutine tests to see if one string is inside another, spaces not withstanding. An example follows. 
sub inside
{
	local($a, $b);			# Make local variables
	($a, $b) = ($_[0], $_[1]);	# Assign values
	$a =~ s/ //g;			# Strip spaces from
	$b =~ s/ //g;			#   local variables
	($a =~ /$b/ || $b =~ /$a/);	# Is $b inside $a
					#   or $a inside $b?
}

&inside("lemon", "dole money");		# true
In fact, it can even be tidied up by replacing the first two lines with 
local($a, $b) = ($_[0], $_[1]);

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