If you want to avoid the shell messing up your arguments, you could try the 'indirect object'-form of exec.

my $cmd = '/usr/bin/tcpreplay'; # or whereever

exec {$cmd} ($cmd, '-p', $pps, '-i', $eth); # and so on...
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Note there is no comma after the first arg to exec. With this form exec will not use the shell and pass the list of args unchanged as argv[] to the command. This means you have to supply a value for argv[0], too (something one might forget coding too much perl...) Hope this helps....