foreach $exch(@exch) { 
    for($i=0; $i<=$#completefile; $i++) { 
        $su_adapter = undef;
        $su_date = undef;
        while($completefile[$i]=~ m/^.+\\(.+)_.+\..+?:(.+?)\s-\s(.+?)\
+s.+?\/$exch\s\|.+::(.*Up)\(\)/g) { 
            $su_adapter=$1; 
            $su_date=$2; 
        }
        print $su_adapter, $su_date;
    }
}
[download]

I disagree with your point about printing since I generally care as much about a failed match as a successful match, and as long as the OP clearly isn't using warnings, printing undefs (with formatting like perhaps a newline) is appropriate. In any case, were I to have coded this, it would look a lot more like

foreach my $exch (@exch) {
    my ($su_adapter, $su_date);
    foreach (reverse @completefile) {
        last if ($su_adapter, $su_date) = /^.+\\(.+)_.+\..+?:(.+?)\s-\
+s(.+?)\s.+?\/$exch\s\|.+::(.*Up)\(\)/;
    }
    if (defined $su_adapter) {
        print "Last match for $exch: $su_adapter, $su_date\n";
    } else {
        print "No match for $exch\n";
    }
}
[download]

In the end, much of this is strictly academic, since this the OP never posted enough code to really understand what they intended.

In your second snippet, you still print the last match of the last file, not the last match of each file.

I disagree with you point about printing

Then why did you proceed by putting the print in an if?

In your second snippet, you still print the last match of the last file, not the last match of each file.

This depends on what is in @completefile, as I previously discussed. I suppose it's also possible the OP wants just one result for all the elements of $exch, in which case the loop nesting should be reversed.

Then why did you proceed by putting the print in an if?

My point was that every iteration should output. If I didn't worry about using warnings, I could have had

foreach my $exch (@exch) {
    my ($su_adapter, $su_date);
    foreach (reverse @completefile) {
        last if ($su_adapter, $su_date) = /^.+\\(.+)_.+\..+?:(.+?)\s-\
+s(.+?)\s.+?\/$exch\s\|.+::(.*Up)\(\)/;
    }
    print "Results for $exch: $su_adapter, $su_date\n";
}
[download]

which reports the same info with no conditional.

You're right, I apologize, I just updated my post with the script, if anyone can take a look again, i'd appreciate it...