my @strings = (
    "Hello world",
    "multi\n
    line
    string",
    "tabbing\talong\twith spaces",
);

foreach my $s (@strings) {
    my $t = $s;
    $t =~ s/\s*//g;
    print "$s -> $t\n";
}

Hello world -> Helloworld
multi

    line
    string -> multilinestring
tabbing	along	with spaces -> tabbingalongwithspaces