In (?<modifiers>:...) <modifiers> (if present) is some mixture of the 'imopsx' modifiers usually found at the end of a regex. '^' is a modifier in this context that means "use the system default modifiers 'd-imsx'". I'm guessing qr does this to get predictable behavior when $regex is embedded in a larger regex.

Consider:

use strict;
use warnings;

my $text  = 'Abcd';
my $match = 'abcd';
my $regex = qr/$match/;

print "No match: '$text', '$match'\n" if $text !~ /$regex/i;
print "/i match: '$text', '$match'\n" if $text =~ /$match/i;
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Prints:

No match: 'Abcd', 'abcd'
/i match: 'Abcd', 'abcd'
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