use strict;
use warnings;

my @contents= <DATA>;

# indexing your content
my %index;
@index{ map((/address=(\d+\.\d+\.\d+\.\d+)/), @contents) }= 0..$#contents;
# beware! will fail if no address in a line!

# retrieving an index
my $i= $index{'127.0.0.1'};
print defined($i) ? $i : "not found";

__DATA__
# SNMP manager 1 settings manager.1.address=127.0.0.1 manager.1.port=162 manager.1.version=2 manager.1.community=public
# SNMP manager 1 settings manager.1.address=127.0.2.1 manager.1.port=162 manager.1.version=2 manager.1.community=public
# SNMP manager 1 settings manager.1.address=127.0.3.1 manager.1.port=162 manager.1.version=2 manager.1.community=public