If you want to know how it works clearly.You print the array @_ inside your subroutine.@_ is an array which contains the arguments passed to subroutine. If you call the subroutine without using &, then it will pass the data given after two subroutine will be passed.In your case, two + 2 2 will be passed as an argument. In the subroutine, you didn't handle anything with that argument.Then, 2 will be returned from that subroutine.That's why 2 gets printed. If you use &, then 2 from the subroutine will be returned and then it added with + 2 given.Then, it will print 4.

use strict;
use warnings;

sub two { print @_; 2 }

print "Two=%d\n", two + 2;
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If you want to have the array structure , use Dumper function which is in Data module.