Quoting issues in the backticks aside, it might be worth the 4 extra lines of code to simply open a pipe from ps and do the winnowing and line counting yourself in your Perl rather than piping to the external commands.

open( my $ps_fh, "-|", "ps -ef" ) or die "Can't open pipe from ps: $!\
+n";
my $load = 0;
while( <$ps_fh> ) {
  $load++ if /somecmnd\.exe$/;
}
close( $ps_fh );
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