cleaner(\$details);   AND   $details = cleaner($details);

my $dirty;
sub cleaner($dirty){
    my $clean = ($dirty =~ s/\n/<br>/g;);
    $clean =~ s/\t/ /g;
    return $clean;
}

##</code><code>##

sub cleaner {
    my($dirty) = @_;
    ...
}

##</code><code>##

    my $clean = ($dirty =~ s/\n/<br>/g;);

##</code><code>##

    (my $clean = $dirty) =~ s/\n/<br>/g;

##</code><code>##

    my $clean = $dirty;
    $clean =~ s/\n/<br>/g;

##</code><code>##

sub cleaner {
    (my $clean = $_[0]) =~ s/\n/<br>/g;
    $clean =~ s/\t/ /g;
    return $clean;
}

$details = cleaner($details);

##</code><code>##

sub cleaner {
    local $_ = shift;
    s/\n/<br>/g;
    s/\t/ /g;
    return $_;
}

$details = cleaner($details);