If it were me, I'd probably use something like Tie::File to get the number of lines:

Counting lines in large files (which presumably these are, hence the need to split them) is a really terrible way to use Tie::File. To quote the author:

There is a large memory overhead for each record offset and for each cache entry: about 310 bytes per cached data record, and about 21 bytes per offset table entry.

The per-record overhead will limit the maximum number of records you can access per file. Note that accessing the length of the array via $x = scalar @tied_file accesses all records and stores their offsets. The same for foreach (@tied_file), even if you exit the loop early.

A simple:

sub countLines {
    my $filename = shift;
    open my $fh, '<', $filename
        or croak("failed to open '$filename' - $!");
    my $count = 0;
    $count ++ while <$fh>
    return $count:
}
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Is far, far (and for very large files; far) more efficient that abusing Tie::File for this. And it is hardly more complex. For very large files, using a larger buffer will save a little more time:

sub countLines {
    my $filename = shift;
    open my $fh, '<', $filename
        or croak("failed to open '$filename' - $!");
    my $count = 0;
    local $/ = \2**26; ## 64MB raise or lower to taste
    $count += tr[\n][\n] while <$fh>;
    return $count:
}
[download]

And File::Split will blow memory if the input file (or combined output file in the case of merge_files()) is larger than memory.

Blind CPANitis serves no one. Teaching it ...