Why?

If you give either interpreter a script in the other language you end up with a syntax error right at the start and especially in case of a Perl script fed to python, the error is RIGHT at the use strict;. Do you really spend time reviewing the script to find out why do you get

  File "d:\temp\findInFiles.pl", line 1
    use strict;
        ^^^^^^
SyntaxError: invalid syntax
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Even if you did find a way to hide the rest of the python script from Perl and getting it to print that message AND a way to hide the rest of the Perl script from python and getting it to print the opposite message, there is no chance you'd consistently include that thing even in your own scripts.

Besides on a well set system, you do not need to specify the interpreter. The right one is used either based on the shebang or the file extension.