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

Note that there are a bunch of easy modifications (as shown in ikegami's post) that can simplify your code and reduce its predilection for bugs.

No. Four problems with your solution:
• undef($var); is less efficient than $var = undef;.
• The print still needs to be in an if in case there was no match.
• Using undef at all is useless (because of previous point).
• That doesn't find the last match of each file. It only finds the last match of one file.

There are a host of problems with my solution. However it solves the posted issue (reporting cached hits) as I understand the snippet. If we assume that @completefile is a file slurp (i.e. @completefile = <$fh>), then this loop will output the last $exch match in the file. If I misunderstand the spec and each line of @completefile represents a file slurp (i.e. local $/; push @completefile,<$fh>), then a solution could be obtained using a g modifier (and likely an m to boot) on the regex and a while loop:

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.