#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dump;

my $file = qq(buff.txt);

open( FH, "+<", $file ) or die "Opening: $!";

my @array = <FH>;

dd \@array;

s/cuke/beer/ for @array;

seek( FH, 0, 0 ) or die "Seeking: $!";

print FH @array or die "Printing: $!";

truncate( FH, tell(FH) ) or die "Truncating: $!";

close(FH);

dd \@array;

__END__

karls-mac-mini:monks karl$ cat buff.txt
foo
bar
nose
cuke

karls-mac-mini:monks karl$ ./inplace.pl 
["foo\n", "bar\n", "nose\n", "cuke\n", "\n", "\n"]
["foo\n", "bar\n", "nose\n", "beer\n", "\n", "\n"]

karls-mac-mini:monks karl$ cat buff.txt
foo
bar
nose
beer