Unless you tell us what's in %Hash_plfdata and $file_name (some sample case where you think something should match is sufficient), we cannot really offer any help beyond "the grep condition presumbly is never true...".

This, for example, works for me (except for the C/C++-style comment in your code, to be precise):

my %Hash_plfdata = ( foo => ["/bar#"] );
my $file_name = "bar";
...

__END__
BASE p4 path:/bar#
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I think I can see the bug in your grep code.

In your first regular expression, you have written:

in other words, you are trying to match a filename, and you have sensibly quoted it, however you have added a hash char to the end. Was this international? A hash won't denote a comment unless you set the /x regexp option.

A few other comments:

You can reduce the 'leaning toothpicks effect' by choosing a different regexp delimiter. eg:

qr:/\Q$file_name\E:i

Also the second regular expression looks to be matching a file extension. If that is what you are doing, you might want to add an anchor for the end of the string. eg:

 /\.plf^/

1. 1) I'd recommend that you use values instead of keys since you're ignoring the key data.
2. 2) Also, be sure to use the $ anchor instead of ^ for the end of string boundary

foreach my $plf_vals (values %Hash_plfdata) {
    my ($base_p4path) = grep {m{/\Q$file_name\E}i && !m{\.plf$}} @$plf
+_vals;
    print "BASE p4 path:$base_p4path\n";
}
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