You need to tell perl where the variable name stops and where the rest of the text begins, using curlies. Without such a distinction in the example below, "$var12" could mean either $var12 or the value given by join( '', $var1, '2' ).

use strict;
use warnings;

my $var1 = 'one';
my $var12 = 'twelve';

print "$var12\n";   # prints 'twelve'
print "${var1}2\n"; # prints 'one2'
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This is discussed in perldata, "Scalar value constructors".