Uh oh ... I wanted to point out that your solution is wrong, found a conter example and ... it broke my code as well :-(

Here is a fixed solution:

@files = qw(model4run1 model5run1);
#@files = qw(model4run1 model2run1 model4run2 model1run1);

my $first = shift(@files);
my $and = $first;
my $or = $first;
foreach (@files) {
    $and &= $_;
    $or |= $_;
}

my $combined = $and ^ $or;
$combined =~ s/[^\x00].*//;

my $prefix = substr($first, 0, length($combined));
print qq{Prefix is "$prefix"};
[download]

Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature

There's another case this will fail for:

@files = qw(model4run1 model2run1 model4run2 abbot);
...
print Jenda(@files),$/;
__DATA__
model4
[download]

antirice    
The first rule of Perl club is - use Perl
The ith rule of Perl club is - follow rule i - 1 for i > 1

Whadaheck?
...
The problem was in the

$combined =~ s/[^\x00].*//;
[download]

I tried to print the variable with</code> print "\$combined=",unpack("H*",$combined)," ($combined)\n"; </code> and was really surprised:

¢¼combined=0c0d060a183672756e33 (”Ļ
”č6run3)
$combined=0a183672756e33 (
”č6run3)
Prefix is "model4r"
[download]

(I'd be even more surprised if this went through correctly.)

Guess what was the problem ... well the .* did not match everything. I forgot that without /s dot doesn't match newlines. So the fix is very simple

...
$combined =~ s/[^\x00].*//s;
...
[download]

Actually thinking about this it would probably be better to change the last three lines to:

$combined =~ /^(\x00*)/;
my $prefix = substr($first, 0, length($1));
print qq{Prefix is "$prefix"};
[download]

I should have tested my code better :-(

Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature

Hmm, well I guess that is what I get for only doing testing on the example array. I guess I misunderstood how the & worked on strings, I thought it did it on a byte/character basis and only on bit basis for numbers. Thanks for finding this counter example.