my $path = "./mail";
my $bkup = "./backup";

open MAILDIR, $path;

for my $mbox ( grep { -f "$path/$_" and -s _ } readdir MAILDIR )
{
    rename "$path/$mbox", "$bkup/$mbox";
    system( "perl -ne 'print if /^From/..-1' $bkup/$mbox > $path/$mbox" );
}