Not true. A first line of "|\n" would also match, for example. Fix:

my @comp = "seed\n";                                # Fix 1 
while ($line = <>) {
   push(@comp, $line);
   print $line if $comp[0] =~ /^\Q$comp[1]\E\z/;    # Fix 2
   shift(@comp);
}
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But why not just use eq?

my @comp = "seed\n"; 
while ($line = <>) {
   push(@comp, $line);
   print $line if $comp[0] eq $comp[1];
   shift(@comp);
}
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Simpler still:

my $last = ""; 
while (<>) {
   print if $_ eq $last;
   $last = $_;
}
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