open FH, '<', $filename or die ("Can't open $filename");
#FH is a "filehandle", and the '<' means "open in read mode" 
#('>' would be write)
while (<FH>) { #the angles around FH mean "read a line"
    work_with_the_line($_); 
    # $_ holds the current line in this case
    # work_with_the_line would be a subroutine defined by you
}
close FH;
#don't forget to close files!