Question

Assuming that I must do this substitution using a single substitution, what is the preferred method to avoid this error:

Use of uninitialized value $2 in concatenation (.) or string at -e line 1.

With this Perl code:

perl -e 'use strict;use warnings;my $str="a";$str=~s/(a)|(b)/$1foo$2/gsmo;'

The goal here is to either print “afoo” or “foob” depending on what $str contains.

I can use no warnings; but then I am worried I will miss other “real” warnings. I also know that using one pattern makes this convoluted but my actual pattern is much more complicated.

If you care the actual replacements are closer to:

#!perl
my $search = q~(document\.domain.*?</script>)|(</head>)~;
my $search_re = qr/$search/smo;
my $replace = q("$1
<script src=\"/library.js\"></script>
$2");
while (<*.tmpl>) {
  my $str = fead_file($_);
  $str =~ s/$search_re/$replace/gee;
}

But even more complicated, basically the above code just reads from a DB to get the search & replace and then does them to the template. Having to run this script twice with every commit would introduce too much overhead, apparently… so says them…

Answer 1

You could:

my $replace = q("@{[$1||'']}
<script src=\"/library.js\"></script>
@{[$2||'']}");

(using // instead of || on 5.10+)