use strict;

# Single quote string, and back-whack the backslashes
# to avoid misinterpretation (by readers, if not by perl).

my $strInputFileName = 'D:\\direcorty\\file.ext'; 

# Globally replace backslashes with four backslashes.
# Note that these all need backwhacking too, since 
# regex acts somewhat like a quoted string.

$strInputFileName =~ s{\\}{\\\\\\\\}g;