Hopefully this makes it clearer:

#           delimiters
#             |  |  |
#             v  v  v
$localdir =~ s/\//\\/g;
#              ^^ ^^
#              |   |
#    search part   replacement
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But for regexes, I would recommend using what I suggested:

$localdir =~ s{\\}{/}g;
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But as I said, using a module for this is even better:

use warnings;
use strict;
use Path::Class qw/foreign_file foreign_dir/;

my $file = foreign_file('Win32', "..\\Hello\\World.txt");
print $file->as_foreign('Unix'), "\n"; # prints "../Hello/World.txt"
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