It doesn't seem to have anything to do with the adding of new patters.

Instead, YAML::Syck's LoadFile() function seems to always add a level of brackets around all regexes it reads from a YAML file, even if they already have outer brackets.

Example:

test-re.pl:

#!/usr/bin/perl

use strict;
use warnings;
use YAML::Syck qw(LoadFile);
use Data::Dumper;

my $yaml = LoadFile( 'test-re.yaml' );
print Dumper($yaml->{patterns}), "\n";
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test-re.yaml:

--- 
patterns: 
  - !!perl/regexp (?^i:^foo.*bar.*baz)
  - !!perl/regexp ^quux$
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Output:

$VAR1 = [
          qr/(?^:(?^i:^foo.*bar.*baz))/,
          qr/(?^:^quux$)/
        ];
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Output when changing use YAML::Syck to use YAML:

$VAR1 = [
          qr/(?^ui:^foo.*bar.*baz)/,
          qr/(?^u:^quux$)/
        ];
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So yeah, seems to be a YAML::Syck particularity. Whether it is a bug is a matter of opinion I guess, since the regexes do behave the same so nothing should break.