With development versions of Perl you can now use the Encode module like so:

    use Encode qw(encode decode);

    my $iso_data=encode('iso-8859-1',decode('UTF-8',$utf8_data));
[download]

The list of encodings that Encode currently supports is given by Encode::encodings():

koi8-r
dingbats
iso-8859-10
iso-8859-13
cp37
iso-8859-9
iso-8859-6
iso-8859-1
cp1047
iso-8859-4
Internal
iso-8859-2
symbol
iso-8859-3
US-ascii
iso-8859-8
iso-8859-14
UCS-2
iso-8859-5
UTF-8
iso-8859-7
iso-8859-15
cp1250
iso-8859-16
posix-bc
[download]

That list is expandable via inp0ut text file found in the ext/Encode/Encode directory in the perl source tar ball distribution.