May i know why $$ changes often?

bash-2.05b$ perl -e'print "$$\n"'                                                                                          
4444236
bash-2.05b$ echo $$
4100348
bash-2.05b$ perl -e'print "$$\n"'
4431936
bash-2.05b$ echo $$;perl -e'print "$$\n"'
4100348
4268078

Notice that 4100348 does not change, as this is your current shell. The other numbers are the PID or the various perl interpreter processes that get started..

try this for current shell pid

typeset -x x=$$ ; perl -e 'print "$ENV{x}\n"'