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    return $ct{'text/plain'} if exists $ct{'text/plain'};
    return $ct{'text/html'} if exists $ct{'text/html'};
    return $parts[0] if $which =~ /text/; 
    return $parts[1] if $which =~ /html/;

    my $ct = $which eq 'html' ? 'text/html' : 'text/plain';
    return $ct{$ct} if exists $ct{$ct};
...

    # Fall back to whatever's first
    return $parts[0];

$ct{$_->content_type} = $_ for @parts;

$ct{$_->content_type} ||= $_ for @parts;