One way (of many) to parse the data would be like this:

#!/usr/bin/perl -w
use strict;
use Data::Dumper;  # this is a core module 
                   # no "installation" is required

my $line = "meter_read energy_consumption n 00_1 34 n 00_2 53 n 00_3 1
+21\n";
#
# this extracts number pairs and puts them into a hash
#
my %hash = $line =~ m/([\d_]+)\s+(\d+)/g;
print Dumper \%hash;

__END__
$VAR1 = {
          '00_2' => '53',
          '00_3' => '121',
          '00_1' => '34'
        };
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