If it's a timing issue, you could just keep trying. Something like

bjmsys("xspec - ${tmp}.xcm", $v);
my $tries = 10; 
until ( open(DAT, "${tmp}xsfit.dat") || --$tries <= 0 ) { sleep 1; }
die ("Could not open file: $!") if ($tries <= 0); 
my @kT=<DAT>; close DAT;
[download]

Not quick, but dirty :)