Here is a way to it, if one of the files fit into your memory. ( untested )

my %h;
open my $fh, "<file1" or die $!;
while ( defined( $_ = <$fh> ) ) {
  $h{$_} = $.;
}

open my $fh2, "<file2" or die $!;
while ( defined( $_ = <$fh2> ) ) {
  if ( exists $h{$_} ) {
    print "line $. is a dup of line $h{$_}\n";
  }
}
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