16:12 >perl -wE "my $string = 'abcde'; $_ = 42; say; $string =~ /(?{ say })/;"
42
abcde

16:12 >perl -v

This is perl 5, version 22, subversion 0 (v5.22.0) built for MSWin32-x64-multi-thread

##</code><code>##

use strict;
use warnings;

my %searches =
(
    bar  => " bar  \n\n",
    bar1 => " bar1  \nw\n",
    baz  => "baz",
    foo  => "  foo  c  \n",
    foo1 => " foo ",
);

my $x;
my $regex = qr{ ^ \s* (??{ $x }) \s* $ }x;

for (sort keys %searches)
{
    $x = $_;
    print "$_: ", $searches{$_} =~ $regex ? 'match' : 'no match', "\n";
}

##</code><code>##

my $regex = qr{ ^ \s* (??{ $_ }) \s* $ }x;