use strict;

my %sequences;

while (<DATA>) {
  chomp;
  $sequences{$_} = $.;
}

for my $seq (keys %sequences) {
  my $rev_compl = get_rev_compl($seq);
  print "$seq matches $rev_compl on line $sequences{$rev_compl}\n"
    if exists $sequences{$rev_compl};
}

sub get_rev_compl {
  my ($seq) = @_;
  $seq =~ tr/GCAT/CGTA/;
  return reverse $seq;
}
1;
__DATA__
GATTACA
TTAGCAA
CATCATC
TTGCTAA
TGTAATC
GATGATG