Whats wrong with: ... s/(?<=__)(A-Z_)+//g;

Nothing (other than not needing the g modifier :-) but as I said, there are a lot of ways to tackle the problem and this particular question piqued my interest. I have tried accessing pos() inside the while loop but I get "Use of uninitialized value" warnings. Doing the same with a match rather than a substitution seems fine and I can even assign to pos() to affect where it matches.

#!/usr/bin/perl -l
#
use strict;
use warnings;

$_ = q{DFR7234C__A_B_C_Bonzo_Dog_D_B};
print;

while ( s{(?<=__)[A-Z]_}{} )
{
    print pos();
}

print;
print q{-} x 25;
# ---------------------

$_ = q{DFR7234C__A_B_C_Bonzo_Dog_D_B};
print;

while ( m{_}g )
{
    print pos();
}

print q{-} x 25;
# ---------------------

$_ = q{DFR7234C__A_B_C_Bonzo_Dog_D_B};
print;

while ( m{_}g )
{
    print pos();
    pos() = 23 if pos() >= 12 && pos() < 26;
}

print q{-} x 25;
# ---------------------
[download]

produces

DFR7234C__A_B_C_Bonzo_Dog_D_B
Use of uninitialized value in print at ./spw644148G line 11.

Use of uninitialized value in print at ./spw644148G line 11.

Use of uninitialized value in print at ./spw644148G line 11.

DFR7234C__Bonzo_Dog_D_B
-------------------------
DFR7234C__A_B_C_Bonzo_Dog_D_B
9
10
12
14
16
22
26
28
-------------------------
DFR7234C__A_B_C_Bonzo_Dog_D_B
9
10
12
26
28
-------------------------
[download]

I agree with you, oha's solution is very nice indeed.

Thank you for your reply,

JohnGG