That's not what I see: 

Win8 Strawberry 5.8.9.5 (32)  Thu 07/15/2021 13:10:20
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
my $file="Billy_Bob_Smith";
$file =~ s/_/\\_/g;
print "'$file' \n";

use Data::Dumper;
print Dumper $file;
^Z
'Billy\_Bob\_Smith'
$VAR1 = 'Billy\\_Bob\\_Smith';

[download]
How are you looking at $file?

Update: Further to Fletch's post:
Utilities like Data::Dumper and Data::Dump try to represent output as "proper" single- or double-quoted strings. A single-quoted string (as in the Data::Dumper example above) handles multiple backslashes that are not before a quote terminal like this: 

Win8 Strawberry 5.8.9.5 (32)  Thu 07/15/2021 13:10:29
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
my $string = 'a\b\\c\\\d\\\\e';
print ">$string< \n";
^Z
>a\b\c\\d\\e<

[download]
You may be seeing an artifact of your data dumper utility, whatever it is.

Give a man a fish:  <%-{-{-{-<

In reply to Re: Extra backlash getting added to string by AnomalousMonk
in thread Extra backlash getting added to string by newperldeveloper

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