Assuming your install script is in perl, the following should work on both systems (Tested OK on Win32):

my $p;
for (split /,|;/, $ENV{PATH}){
      m/\Wperl\W/i and $p=$_
    };

print qq[===$p==\n]; # $p contains the path to perl
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Admittedly, if you have perl-like paths in the PATH, this code would be confused - but then you could get fancy, and try to verify existance of perl, and it's execute-ability.

Update:Ignore this drivel. Use ikegami's (++) correct answer below.