If I understand your question correctly, you'll need to execute system singularly, and build your loop around it, as opposed to in it

foreach ($firstbuild, @otherbuilds)
{
    system (qq(C:/execu $_ file.c));
}
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One thing you may consider, esp. if @otherbuilds can grow to be sufficiently large: system is a double fork, in terms of processes spawned. One proc. for the system call, one for the actual command to run. If you're going to be calling system on more than a couple of items, you may consider doing something like so:

open (OUT, ">cmd.txt") || die "$!\n";
foreach ($firstbuild, @otherbuilds)
{
    print OUT "C:/execu $_ file.c\n";
}
close OUT;
system ("cmd.txt");
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The reason? Instead of spawning 2N processes, you'll spawn only N+1. 3 or 4 system calls probably doesn't warrant the extra work. If you got 100 or so...well, you get the picture.

ÅßÅ×ÅßÅ "It is a very mixed blessing to be brought back from the dead." -- Kurt Vonnegut