If you especially don't want the shell interpreting shell metacharacters, then don't pass the command to the shell, use the 4+ argument form of open.

open(CMD, '-|', '/usr/bin/ls', '$4', '$PATH');
my $output = do { local $/; <CMD> };
close CMD;
print "$output\n";
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Here is the output, you'll see that the "$4" and "$PATH" were not interpreted as variables before being sent to ls.

ls: 0653-341 The file $4 does not exist.
ls: 0653-341 The file $PATH does not exist.
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