my $max_source_length = ( sort {$b<=>$a} keys %source_lengths )[0];

for my $key ( sort {$a<=>$b} keys %distrbtn ) {
    my $size = $key - 3;
    my $freq = $distrbtn{$key};

# find the first set of source strings of equal or greater size:

    my $source_key = $key;
    while  ( $source_key <= $max_source_length and
             not exists( $source_lengths{$source_key} )) {
        $source_key++;
    }
    if ( $source_key > $max_source_length ) {
        die "We can't do this: strings from $input2 aren't long enough";
    }
    ...
}