$? == 256 means the child called exit(1) (256>>8 = 1). Chances are that the value (1) is not significant beyond meaning an error occurred. You'd think it would have emitted a message to STDERR identifying the error.

$ perl -le'qx{date abc}; print $?'
date: invalid date `abc'
256
[download]

One likely cause is improper escaping of arguments. Print out $cmd, check for bad quoting, and try that same command at the prompt.

$ perl -le'qx{cat some file}; print $?'
cat: some: No such file or directory
cat: file: No such file or directory
256

$ perl -le'qx{cat "some file"}; print $?'
0
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You mentioned the child is some kind of searching utility. An error could also indicate no matches were found.

$ perl -le'qx{echo foo | grep bar}; print $?'
256

$ perl -le'qx{echo foo | grep foo}; print $?'
0
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The program's man page might identify situations where it might return an error.