#!/usr/bin/perl
use warnings;
use strict;
use Syntax::Construct qw{ \K };

while (<DATA>) {
    s/ (?:clothes|something) # Starting keyword.
       \K                    # Don't include the previous part into the replacement.
       (.*?)                 # The part to be quoted.
       (?=\.m)               # Followed by ".m".
     /'$1'/xg;
    print;
}

__DATA__
abc clothes doing shopping at the .m abc something doing shopping at the .mabc