use strict;
use warnings;

my $str = <<TEXT;
First line
1234X
last line
TEXT

print "Matched\n"       if $str =~ /^\d{4,4}[A-Z0-9]$/;
print "Multi-matched\n" if $str =~ /^\d{4,4}[A-Z0-9]$/m;

##</code><code>##

Multi-matched