foreach (keys $db_results) {
    my $match = $1 if $_ =~ /(BLAH\d{2}-\w)/;
    $db_results->{$1} = delete $db_results->{$_};                     
}
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The  $match variable isn't used in the quoted code, so I assume this is a cut-down fragment of working code in which it is subsequently used. If so, be aware that the usage
    my $scalar = $whatever if ... ;
(conditional definition/initialization of a lexical) is a pre-state (5.10+) hack used to create a bastard state-like variable; it has all kinda weird side-effects and is officially Frowned Upon. Avoid it if you can — and you can, e.g.:
    my ($match) = $_ =~ /(whatever)/;
    do_something_with($match) if defined $match;
or (assuming you're in a loop)
    my ($match) = $_ =~ /(whatever)/;
    next unless defined $match;
    ...