Many shells have a specific command-line option to run single commands. Looking at various resources that Google gives, I can't see one for VxWorks, but you might have better luck asking on a VxWorks site. All the hints I found were about running within the shell, not feeding a command from the outside.

It could be that the VxWorks shell reads commands from its standard input. If it does then you could pipe commands into it, for example:

open(my $Vx, '|-', 'windsh') or die "windsh; $!";
print $Vx "Some command\n";
close $Vx;
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See also perlipc
Alternatively you might be able to write the commands to a script file and:

my @output = qx(windsh < scriptfile);
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But note, this is just something you might try, chances are that VxWorks does not support what you want to do.