I want to remove the trailing slash in the path but the code print nothing. Why?

Added exactly one line to your code and got results. Here's the new code:

#!/usr/bin/perl -w 
use strict;
use warnings;
sub main
{
    my $path = "\\\\127.0.0.1\\c$\\bak\\";
    $path =~ s/\\$//;
    print __FILE__.__LINE__." path $path\n";
    return;

}

main();
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and here are the results:

$ ./hackslash.pl
Use of uninitialized value $\ in concatenation (.) or string at ./hack
+slash.pl line 6.
./hackslash.pl8 path \\127.0.0.1\ak
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What I think you really wanted was to add a backslash before the $ since normally that would have been interpoloated resulting in :

# note the backslash in front of the dollar sign
my $path = "\\\\127.0.0.1\\c\$\\bak\\";
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giving you:

$ ./hackslash.pl 
./hackslash.pl8 path \\127.0.0.1\c$\bak
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Creating a sub without calling it will act as a NOOP.