#!/usr/bin/perl
print diff(shift)."\n";
sub diff
{
    my $_ = shift; # I make this local since I'll be making
                   # recursive calls
    # I'll just include addition:
    return diff($1)."+".diff($2) if m/(.+?)\+(.+);
    ... # other differentiation rules here
}

##</code><code>##

...
if (m/(.+?)\+(.+))
my $firstTerm = $1;
my $secondTerm = $2;
return diff($firstTerm)."+".diff($secondTerm);

##</code><code>##

sub diff
{
    my $_=shift; my $1; my $2;
    ...
}

##</code><code>##

Can't use global $1 in "my"...
Execution ... aborted due to compilation errors.