in reply to (OT) Perl command line interpolation

This is not really a perl question - the shebang line is used by the shell to determine which command will handle the script.

env is a binary that will execute your command in a modified environment -- man env is your friend!

#!/usr/bin/env -i perl # executes perl in an empty environment - no PERL5ENV etc #!/usr/bin/env -i PERL5LIB=~/src perl # executes perl in with only specified PERL5ENV etc
Using env to run perl might be very slightly slower - but I doubt you will be able to measure the impact.