I think in some(?) Win32 systems you can set $ENV{INTRALINK_DEFAULT_USER}, and it might work (don't have a windows system running right now). On unix-type systems the variable will only be seen by processes forked from your perl program (so, also for processes created with system and friends).

If you want to set an environment variable, you might be better off sourcing some shell/DOS script at the start of your session / in autoexec.bat. See also perldoc -q env