IIRC, PBP deprecates the use of constant in favor of the Readonly module, which gives you (among other things) read-only scalars which interpolate as expected in double-quotish things like strings and regexes.

I can't test this right now, but I think the following works and also gives you a more elegant regex interpolation:

use warnings;
use strict;

use Readonly;

Readonly my $BAR => qr{ bar }xms;

my $string = q{fie foo bar baz barf};

print qq{found $1} if $string =~ m{ foo \s* ($BAR) \s* baz }xms;
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Should print:

found bar
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