First off, a potentially easier solution in a long-term sense may be to migrate the scripts you are calling into modules. This is of course not the question you asked, but something to consider.

Another possibility is reading in your script and using do to execute the code. See How to read code from file and execute it in loop ?.

Since you are trying to capture the output from another process, backticks should do what you want. It returns a string, so you should be assigning it to a scalar, but that should not have interfered with your syntax. Are you sure your success.pl script outputs to STDOUT? I suspect it's actually writing to STDERR, which still gets output to the screen on normal execution but is not captured by backticks. What happens when you try again, this time redirecting STDERR to STDOUT:

my $var =  `success.pl 2>&1`;
print $var;
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Update: This, of course, only works if you can run that command on the command line, i.e. your script is set executable. Otherwise, you should type:

my $var =  `perl success.pl 2>&1`;
print $var;
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See Quote Like Operators.