Why not check the return value of spawn?

$test=Expect->spawn("/bin/sh")  || die "Can't spawn /bin/sh:$!";
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On a side note, shell prompts are completely configurable, they can go from:

#
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to

[ user@host: 10:15pm: 12 ] ~/docs >
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You'll need to be more specific as what you think the prompts will look like to generate a better regex.

HTH