$data_string = "hallo test und so weiter";

$string_ohne_leerzeichen = substr($data_string, $pos-1, $laenge) =~ s/\s//g;

my $foo = "bar bar";
$foo = $foo =~ s/\s//g;

#!/usr/bin/perl

use strict;
use warnings;

my $foo = "Hallo Test hallo";
$foo =~ s/\s//g;
print $foo;

#!/usr/bin/perl

use strict;
use warnings;

my $foo = "Hallo Test hallo";
(my $neue_var = $foo) =~ s/\s//g;
print $foo;

#!/usr/bin/perl

use strict;
use warnings;

(my $foo = "Hallo Test hallo") =~ s/\s//g;
print $foo;