Two approaches, one simple, one less so...

my $v = "lech_lecht";

#APPROACH ONE - single regex
$cnt = 0;
$v =~ m{
    .*? 
      (?{$cnt++})
    lecht
      (?{$cnt+=4})
}x;
print "$cnt\n";


#APPROACH TWO - feed regex output to length()
$v =~ /.*?lecht/;
print length $&, "\n";

__OUTPUT__
10
10
[download]

Note, that for position, you may want to subtract one from the result, as these approaches give 1-based answers.

,welchavw