#!/usr/bin/perl
use feature 'say';
use strict;
use warnings;
my $str = "vs or Vs or vs. or VS. or etc avs and metch";
say
$str =~ s/\b(vs
|etc
)\b\
.?/\L$1./gir
;# => vs. or vs. or vs. or vs. or etc. avs and metch
IyEvdXNyL2Jpbi9wZXJsCnVzZSBmZWF0dXJlICdzYXknOwp1c2Ugc3RyaWN0Owp1c2Ugd2FybmluZ3M7CgpteSAkc3RyID0gInZzIG9yIFZzIG9yIHZzLiBvciBWUy4gb3IgZXRjIGF2cyBhbmQgbWV0Y2giOwpzYXkgJHN0ciA9fiBzL1xiKHZzfGV0YylcYlwuPy9cTCQxLi9naXI7CiMgPT4gdnMuIG9yIHZzLiBvciB2cy4gb3IgdnMuIG9yIGV0Yy4gYXZzIGFuZCBtZXRjaA==