You are probably running an elderly version of Perl. What do you get when you run /usr/local/bin/perl -v on the command line? The three-argument form of open was introduced in Perl 5.6 according to perl56delta as were lexical filehandles (the my $fh). If you are running an earlier version then change

open(my $fh, "<", "data.txt") || die "Can't open file: $!";
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to

open (FH, '<data.txt') || die "Can't open file: $!";
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and

while(my $line = <$fh>) {
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to

while(my $line = <FH>) {
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You may want to consider upgrading your version of Perl as 5.005 is positively ancient.

Cheers,

JohnGG