As always, There Is More Than One Way To Do It. If you're certain that the grep condition will always match exactly one element of @files:

• $ScontrolFile = (grep {/_CF_/} @files)[0]; - create a "temporary" list and get its first element
• ($ScontrolFile) = grep {/_CF_/} @files; - use assignment in list context
• use List::Util qw/first/; $ScontrolFile = first {/_CF_/} @files; - use first from the core module List::Util, which has the advantage that it stops looking once it finds a match

However, none of the above solutions provide protection against the case that @files does not contain exactly one string matching the pattern, which is why I suggested the extra array. <update> AnomalousMonk showed a way to add the check to the second of my examples above. To add a check to the other two, </update> you could theoretically search the array a second time, using grep in scalar context (if ( grep({/_CF_/} @files)!=1 ) { die ... }), but that's fairly wasteful and you'll have duplicated code (the grep block). Of course, you can also write an explicit loop:

my $ScontrolFile;
for my $file (@files) {
    if ( $file =~ /_CF_/ ) {
        die "more than one _CF_ file found in: @files"
            if defined $ScontrolFile;
        $ScontrolFile = $file;
    }
}
die "_CF_ file not found in: @files" unless defined $ScontrolFile;
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Update: Added check for >1 file in the above code, and see update in text.