You might like to consider this example code using inplace edit:

use warnings;
use strict;

# Create test file
open testOut, "> test.txt";
print testOut $_ while (<DATA>);
close testOut;

# Get "user" edits
my @userEdits =
  (
  "Manager 1: IP 127.0.0.1" ,
  "Manager 10: IP 127.0.0.10",
  "Manager 4: IP 127.0.0.40",
  "Manager 7:",
  );

# create update hash from user input
my %edits;
for (@userEdits)
  {
  my ($key, $value) = /(\d+):(?:.*?((?:\d{1,3}\.){3}\d{1,3}))?/;
  $edits{$key} = [$value, 0]; # value and written flag
  }

# Update the file
local $^I = '.bak';
local @ARGV = ('test.txt');
my ($key, $value);

while (<>) # inplace edit
  {
  ($key, $value) = /(\d+):.*?((?:\d{1,3}\.){3}\d{1,3})/;
  my $line = $_;
  if (defined $key && exists $edits{$key})
    {
    next if ! defined $edits{$key}->[0]; #delete file entry
    $line = "Manager $key: IP $edits{$key}->[0]\n";
    ++$edits{$key}->[1]; # Mark as written
    }

  print $line;
  }
  continue
  {
  if (eof)
    {
    for (sort keys %edits)
      {
      next if ! defined $edits{$_} || ! defined $edits{$_}->[0];
      print "Manager $_: IP $edits{$_}->[0]\n" if ! $edits{$_}->[1];
      }
    last;
    }
  }

__DATA__
Manager 1: IP 127.0.0.1
Manager 3: IP 127.0.0.3
Manager 4: IP 127.0.0.21
Manager 7: IP 127.0.0.12
[download]

Manager 1: IP 127.0.0.1
Manager 3: IP 127.0.0.3
Manager 4: IP 127.0.0.40
Manager 10: IP 127.0.0.10
[download]