What determines which shell Perl will use as default when passed a command in backticks?

Perl will use /bin/sh. You might consider using ksh's -c switch to run the command.

system('/usr/bin/ksh', '-c', 'ls -l');    # For example.

On closer inspection though, it looks like you are trying to do that. I'm not sure exactly what you are asking. What you have won't do though. You might want to have a look at IPC::Open2 and Expect.

As for eliminating the need to backwhack everything, read perldoc -f quotemeta.

Update: The backticks in this snippet of code aren't doing anything for you:

$cmds[0] = "`su $user -c \"ksh -c $action \"`";

That's the same as if you typed `su THE_USER -c "ksh -c THE_ACTION"` at the shell prompt. This would be a lot easier to read:

$cmds[0] = qq(su $user -c "ksh -c $action");
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Also, if this is a short script and you just need to bang something out, you might find its easier just to call the command with system rather than trying to open a pipe from it. It'll do the right thing with your file descriptors so that you can enter the password and read or redirect the output.

perl -e '$user="whoever"; $action="whatever"; system "su", $user, "-c"
+, "ksh -c $action");'
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-sauoq
"My two cents aren't worth a dime.";