hi
Use a total counter outside your loop and print it out at
the end.
Something along the lines of:

 use strict;
 use warnings;

 my $total;

 while(<DATA>) {
    my ($county, $year, $yr_tot) = split;
    $total += $yr_tot if $county =~ /26005/;
 }
 print "Total for county 26005: $total\n";

__END__
26005 2004 1
26005 2004 2
26005 2004 3
26004 2004 1
26003 2004 1
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si_lence
update: typo