You are using backticks which capture program output and storing the result in $cmd. You never print that output therefore $output is empty.

use warnings;
use strict;
use IO::Scalar;
my $output;

tie (*STDOUT, 'IO::Scalar', \$output); # Use this to capture STDOUT
print `echo foo`;
untie *STDOUT; # Use this to return STDOUT to normal
print "my output='$output'"
__DATA__
my output='foo
'
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