'use' gets executed out of order. You'll either need to switch to 'require', place it in a BEGIN block, or set it before you run the script:

$ perl -e 'use LWP; print LWP::Protocol::implementor("https"),"\n";'
LWP::Protocol::https
$ perl -e 'use LWP; $ENV{"PERL_LWP_USE_HTTP_10"}=1; print LWP::Protoco
+l::implementor("https"),"\n";'
LWP::Protocol::https
$ perl -e '$ENV{"PERL_LWP_USE_HTTP_10"}=1; use LWP; print LWP::Protoco
+l::implementor("https"),"\n";'
LWP::Protocol::https
$ perl -e '$ENV{"PERL_LWP_USE_HTTP_10"}=1; require 'LWP'; print LWP::P
+rotocol::implementor("https"),"\n";'
LWP::Protocol::https10
$ perl -e 'BEGIN { $ENV{"PERL_LWP_USE_HTTP_10"}=1; } use LWP; print LW
+P::Protocol::implementor("https"),"\n";'
LWP::Protocol::https10
$ export PERL_LWP_USE_HTTP_10=1; perl -e 'use LWP; print LWP::Protocol
+::implementor("https"),"\n";'
LWP::Protocol::https10

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In reply to Re^3: Forcing HTTP/1.0 and mangling headers by jhourcle
in thread Forcing HTTP/1.0 and mangling headers by yam

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