The regex answers already given are the best, most "Perlish" solutions.
Here are three other ways to remove or replace a single character, using index and substr.

my $data1 = "abc\ndef";
my $data2 = "foobar\nbaz";
my $data3 = "Albus\nPercival\nWulfric\nBrian\nDumbledore\n";

my $pos1 = index($data1, "\n"); # $pos1 is 3
my $pos2 = index($data2, "\n"); # $pos2 is 6
my $pos3 = index($data3, "\n"); # $pos3 is 5

# Put together the part before $pos1,
#          and the part after  $pos1.
$data1 = substr($data1, 0,      $pos1)
       . substr($data1, $pos1+1      );

# Tell the part of the string that contains
# the "\n" to resize itself to nothing, as an lvalue.
substr($data2, $pos2, 1) = '';

# Replace the part of the string that contains
# the "\n" with three spaces.
substr($data3, $pos3, 1, '   ');

use Data::Dumper;
$Data::Dumper::Useqq = 1;
print Dumper($data1, $data2, $data3);
# prints:
#  $VAR1 = "abcdef";
#  $VAR2 = "foobarbaz";
#  $VAR3 = "Albus   Percival\nWulfric\nBrian\nDumbledore\n";
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