Another way is to separate the variables:

use warnings;
use strict;

my ($userinput1, $userinput2);
$userinput1 = <STDIN>;  #first name
chomp $userinput1;
$userinput2 = <STDIN>;  #last name
print "Hello $userinput1", '_', $userinput2;
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• I also fixed your typo: $userinputer2
• You need to chomp to get rid of the newline