I hope I am understanding your question correctly. This little piece of code takes a string delimited by spaces, splits it up into a list, and exits with a true flag if any one element of the list contains exactly one letter followed by exactly one digit.

use strict;

my $string = 'bar a2 b2 c3 d3 d4 a1 f2 f3';
my $flag = 0;

foreach(split(/\s+/, $string)) {
   $flag++, last if /\w\d/;
}
print "foo's found in list\n" if $flag;
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If you wanted to specify a list of foo's to match against, such as only d4 or b2, you could try this instead . . .

use strict;

my @foos = qw(d4 b2);
my $string = 'bar a2 b2 c3 d3 d4 a1 f2 f3';
my $flag = 0;

foreach my $cand (split(/\s+/, $string)) {
   foreach(@foos) {
       $flag++, last if $cand =~ /$_/;
   }
}
print "foo's found in list\n" if $flag;
[download]

Hope this helps, Jeff

UPDATE: Oops, you wanted d4 AND b2 - luckily for me, merlyn has got you covered . . .