It has to do with how foreach (and for, which is an exact synonym) works. foreach will construct the entire list, then iterate through it. This can be very memory-intensive, which will slow the processing speed (due to cache misses and virtual memory issues.) A nearly exact rewrite of foreach in terms of while would look something like:

foreach my $n (<*.*>)
{
    # do stuff
}

----

my @list = <*.*>;
my $i = 0;
while ($i <= $#list)
{
    my $n = $list[$i];
    # do stuff
}
continue
{
    $i++;
}
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