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#!/usr/bin/perl
print diff(shift)."\n";
...
    return diff($1)."+".diff($2) if m/(.+?)\+(.+);
    ... # other differentiation rules here
}

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

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

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