This may help:

#!/usr/bin/perl -l -w
use strict;

my $str1 = "abc";
my $str2 = "abcde";

my @arr = qw(a b c);
my $chars = join '', @arr;
my $re = qr(^[\Q$chars\E]+$);

print "hello1" if $str1 =~ $re;
print "hello2" if $str2 =~ $re;
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