my $subst = 'lc $1';
# and later
$str =~ s{$pattern}{push @pos, pos $str; eval $subst; }eg


##</code><code>##

use strict;
use warnings;
use Data::Dumper;

my @pos;
my $str = "foobarbaz";

if ($str =~ s{([aeiou]+)(?{ push @pos, pos $str })}{<--$1-->}g){
	print Dumper \@pos;
	print $str, "\n";
}