open FIN, ... or die ...;
open FOUT, ... or die ...;
while($line = <FIN>)
{
  $line =~ s/somepattern/somethingelse/g;
}
close FIN;
close FOUT;

##</code><code>##

perl -ne "s/somepattern/somethingelse/g" infile > outfile

##</code><code>##

setVarAndDoSomething(varname1, value1);