use strict;
use warnings;
use Data::Dumper;
use utf8;

my $str = " 1 i á \x{3C3} _ ";          # \x{3C3} = small sigma 
warn Dumper $str;

$str =~ s/\w+//g;                       # delete all alpha-nums
warn Dumper $str;

warn "WORKS!" if $str =~ m/^ +$/;

##</code><code>##

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/utf8.pl 
$VAR1 = " 1 i \x{e1} \x{3c3} _ ";
$VAR1 = '      ';
WORKS! at d:/tmp/pm/utf8.pl line 12.