$ cat xx
#!/usr/bin/perl

use strict;
use warnings;

foreach my $x (["foo", "bar"], ["fo", "obar"]) {
    my ($foo, $bar) = @$x;
    "" =~ /$foo/;
    "" =~ /$foo$bar/;
}
 
__END__
$ perl -Dr xx 2>&1 | 
  perl -nle 'if (/EXECUTING/ .. eof) {print if/^Compiling/}'
Compiling REx `foo'
Compiling REx `foobar'
Compiling REx `fo'