use Fcntl qw( &SEEK_SET );

open(FH, "+<foo.out")
  or die "failed to open file";

while (<FH>) {
  my $line = $_;
  chomp $line;

  $line =~ s/foo/f/g;

  push @output, $line;

}

my $output = join("\n", @output);

seek(FH, 0 , SEEK_SET );

print FH $output, "\n";

truncate(FH, length($output) );

close(FH);

exit 0;