This works on your data:

#/usr/bin/perl -w
use strict;

my @idx;
my @countries;

open B, '< db.txt'
  or die $!;

my $header = <B>;
pos($header) = 1 + index $header, ':';

while ($header =~ /(\w+)/g) {
    push @idx, pos($header) - length($1);
    push @countries, $1;
}

while (<B>) {
    my ($fruit) = /\s([a-z]+)/;
    foreach (my $i = 0; $i < @idx; $i++) {
        print "$fruit missing for $countries[$i]\n"
          if length() <= $idx[$i]
             or substr($_, $idx[$i], length($countries[$i])) =~ /^\s+$
+/;
    }
}
[download]

Output:

pears missing for Canada
watermelons missing for US
watermelons missing for Taiwan
[download]