use MIME::QuotedPrint;

my $msg = <your string>;
$msg =~ s/=\?ISO-8859-1\?Q\?//g; # Cut a charset information and a criterion of quoted print string.
$msg =~ s/\?=//g; # Cut a criterion of quoted print string end. 
$msg = MIME::QuotedPrint::decode($msg); # Decode string