open my $in, '<:raw', $filename or die "Can't read $filename: $!\n";
if ($filename=~/\.gz$/) {
    $in= new IO::Uncompress::Gunzip $in, { AutoClose => 1 };
}
# later on
$_= <$in>;
chomp;
@headers= split /\t/, lc decode('UTF-8' => $_);
# That's not really required as the header will always be
# ASCII-characters… But for completeness sake…

# further down I have a loop
while (<$in>) {
    chomp;
    # …
    @line{@headers}= split /\t/, decode('UTF-8' => $_);