a little trick. (Still uses regexes, but nothing complicated, not even any captures.)

sub find_runs
{
    local $_ = shift;
    $_ ^= substr $_, 1; # string shifted left one char.
    my @r;
    push @r, [ $-[0], $+[0] - $-[0] + 1 ] while /\x00+/g;
    @r
}

# find the 'oo' and the 'ttt'
my $s = "look for doubled lettters";
for my $r ( find_runs($s) )
{
    print substr $s, $r->[0], $r->[1];
}
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