Now a Windows solution ...
At least, if you use ActiveState Perl you can set in the registry this key:
HKEY_LOCAL_MACHINE\Software\Perl\lib to the path you can add permanently to @INC.
Example:

REGEDIT4

[HKEY_LOCAL_MACHINE\SOFTWARE\Perl]
@="C:\\Perl"
"bindir"="C:\\Perl\\bin"
"lib"="c:\\perl\\site\\mylib"
[download]

Greets
Hopes