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.
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