Yeah, on the DOS or UNIX command line, the "-pi.bak -e" options have the effect of considerably simplifying the syntax for modifying a text file.

If you want to write a real script, it is slightly more complicated, those simplifications no longer apply.

Within a Perl script, you would need to do something like this;

#!/usr/bin/perl;
use strict;
use warnings;

my $file_in = shift;
open my $FH, "<", $file_in or die "cannot open $file_in $!";
while (<$FH>) {
     s/test/replace/g;
     print;
}
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The above might still be considered simplistic, it is just giving an idea. Please don't hesitate to ask if you need further information.