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my $s = 'ab';
my $pat = qr/^(a{1,3})(?=b)/;
...
{
    $s =~ s/$pat/$1a/g;
}

my $s = 'ab';
my $pat = qr/^(a{1,3})(?=b)/;

do{ $s =~ s/$pat/$1a/g } while $s =~ m/$pat/;