#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $data = do{local $/; <DATA>};

print Dumper([$data]), "\n\n";

my $result_1;
for (split /(?<=\n)/, $data) {
    next unless /^\Qsnmp-server host\E/;
    $result_1 .= $_;
}
print "---Result 1---\n$result_1\n\n";

my $result_2;
for (split /(?<=\n)/, $data) {
    next unless /^\Qsnmp-server\E/;
    s/^((?:\S+\s+){3})\S+/${1}Bob/;
    $result_2 .= $_;
}
print "---Result 2---\n$result_2\n\n";

__DATA__
snmp-server host 10.234.171.45 admin
snmp-server host 10.234.171.46 admin
snmp-server host 10.234.171.51 admin  frame-relay config snmp
snmp-server host 10.234.171.75 admin
snmp-server host 10.234.171.51 publ  frame-relay config snmp
snmp-server host 10.234.171.54 publ  frame-relay config snmp
snmp-server host 10.234.171.59 publ
!snmp-server host 10.234.171.46 admin
snmp-server host 10.234.171.51 admin  frame-relay config snmp
snmp-server host 10.234.171.75 admin
snmp-server host 10.234.171.51 publ  frame-relay config snmp
snmp-server host 10.234.171.54 publ  frame-relay config snmp
snmp-server host 10.234.171.59 publ
!snmp-server host 10.234.171.51 admin  frame-relay config snmp
snmp-server host 10.234.171.75 admin
snmp-server host 10.234.171.51 publ  frame-relay config snmp
snmp-server host 10.234.171.54 publ  frame-relay config snmp
snmp-server host 10.234.171.59 publ
!snmp-server host 10.234.171.75 admin
snmp-server host 10.234.171.51 publ  frame-relay config snmp
snmp-server host 10.234.171.54 publ  frame-relay config snmp
snmp-server host 10.234.171.59 publ
!snmp-server host 10.234.171.51 publ  frame-relay config snmp
snmp-server host 10.234.171.54 publ  frame-relay config snmp
snmp-server host 10.234.171.59 publ
!snmp-server host 10.234.171.54 publ  frame-relay config snmp
snmp-server host 10.234.171.59 publ
!snmp-server host 10.234.171.59 publ
!