You forgot to take into account that $cmd contains double quotes. Why didn't you try it?

use strict;
use warnings;
my $cmd = '/opt/sudo/sudo /usr/bin/su - $ENV{LOGNAME} -c "$someScript"
+';
$cmd = eval "\"$cmd\"";
print($cmd, "\n");
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outputs

Scalar found where operator expected at (eval 1) line 1, near ""/opt/s
+udo/sudo /usr/bin/su - $ENV{LOGNAME} -c "$someScript"
        (Missing operator before $someScript?)
String found where operator expected at (eval 1) line 1, near "$someSc
+ript"""
        (Missing operator before ""?)
Use of uninitialized value in print at 574310.pl line 5.
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