Works for me.

I'd say your file's encoding is not what you think it is.

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 $str =~ m/^ +$/;

[download]

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.

[download]

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

*) PM has problems displaying unicode characters like "σ" inside code tags

update

In reply to Re: UTF8 versus \w in pattern matching (basic test) by LanX
in thread UTF8 versus \w in pattern matching by mldvx4

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