use utf8; # All hard-coded strings will be assumed to be UTF-8
my $temp = encode( "iso-8859-1", 'Köln' );
...

##</code><code>##

my $temp = encode( "iso-8859-1", decode("UTF-8", 'Köln') );
...

##</code><code>##

# Files (change input encoding to match file encoding):
open my $F, "<:encoding(UTF-8)", "myfile" or die "Error reading myfile: $!";
my $line = <$F>;  # $line contains a decoded string
say encode( "iso-8859-1", $line );

# Command-Line args:
my $arg = decode("UTF-8", $ARGV[0]);

# Or, command-line args is an appropriate use of Encode::Locale
use Encode::Locale;
my $arg = decode("locale", $ARGV[0]);