There is no way that you should using 100% cpu with 10 threads performing IO
But it's not just IO, LWP::UserAgent loads and parses several Perl modules on demand.

For instance, running the OP script (with a fixed set of URLs) under strace on my machine shows that every process reads... 

bytes
Compress::Raw::Zlib
Compress::Zlib
Fcntl
File::Glob
File::GlobMapper
File::Spec
File::Spec::Unix
HTML::Entities
HTML::HeadParser
HTML::Parser
IO
IO::Compress::Adapter::Deflate
IO::Compress::Base
IO::Compress::Base::Common
IO::Compress::Gzip
IO::Compress::Gzip::Constants
IO::Compress::RawDeflate
IO::Compress::Zlib::Extra
IO::File
IO::Handle
IO::Seekable
IO::Select
IO::Socket
IO::Socket::INET
IO::Socket::UNIX
IO::Uncompress::Adapter::Inflate
IO::Uncompress::Base
IO::Uncompress::Gunzip
IO::Uncompress::RawInflate
List::Util
LWP::Protocol::http
Net::HTTP
Net::HTTP::Methods
Scalar::Util
SelectSaver
Socket
Symbol
URI::_generic
URI::http
URI::_query
URI::_server
utf8

[download]

In reply to Re^2: Parallel::ForkManager (high cpu and a lot of memory) by salva
in thread Parallel::ForkManager (high cpu and a lot of memory) by marto9

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