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;