use strict;
use IO::File;

my $f = new IO::File "filetoread.something", "r"
           or die "Can not open input file";
while (<$f>)
{
  # do stuff ...
}

undef $f; # close the file
          # note that you don't even have to do this
          # this is optional because the file will be
          # closed when Perl script finishes. I am just
          # forcing an early file closure here.