use IO::File qw( &SEEK_SET );
 
my $fh = IO::File->new("+<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);
 
$fh->seek( 0 , SEEK_SET );
 
$fh->print($output, "\n");
 
$fh->truncate( length($output) );
 
$fh->close;
 
exit 0;

##</code><code>##

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;