I think what you want is to process some file and replace some regex with another regex?

If so, then consider this code.

die "Usage $0 remove_regex replace_regex" if (@ARGV !=2);
my ($remove, $replace) = @ARGV;

while(<stdin>)
{
   s/$remove/$replace/g;
   print;
}
[download]

@ARGV should be considered a "read only" variable like in other languages.
Here use like: replace.pl a b <infile >outfile
If you want to pass filenames on command line, then same idea with getting values of @ARGV, but you will have to use a open() statement to read or write them.