[sf-perl] dynamic #! line

[yary]

Depending on the situation, you might be able to eliminate the need
for the shebang by calling perl explicitly, that is, use

/path/to/perl /usr/local/bin/myscript -arg foo ...

instead of

/usr/local/bin/myscript -arg foo ...

when you need to run "myscript".

Works if you get to write the install instructions for "myscript", and
can depend on people following them.