Your issue is arising because the first variable Perl is trying to interpolate is read as $userinput1_. You can avoid this issue by wrapping your variable name in curly brackets:

#!/usr/local/bin/perl
use strict;
use warnings;
my ($userinput1, $userinput2);
$userinput1 = <STDIN>;  #first name
$userinput2 = <STDIN>;  #last name
print "Hello ${userinput1}_$userinput2";
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Note I fixed a typo in your variable names. As well, given your intended output, you likely want to chomp your inputs:

#!/usr/local/bin/perl
use strict;
use warnings;
my ($userinput1, $userinput2);
chomp($userinput1 = <STDIN>);  #first name
chomp($userinput2 = <STDIN>);  #last name
print "Hello ${userinput1}_$userinput2";
[download]