Perhaps?

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

my @array = ("x y.g z 123", "a b.f c 456", "d b.c w 321");

my @include = ("b","q");
my %include = map { $_ => 1 } @include;

my @keep;

foreach my $row (@array)
{
    my $important_letter = ($row =~ m/\s+([a-z])\./)[0];    
    push (@keep, $row) if ($include{$important_letter});
}

foreach (@keep)
{
   print "keep: $_\n";
}
__END__
keep: a b.f c 456
keep: d b.c w 321
[download]

I would keep things simple. You need some kind of looping statement. Then something to figure out whether this is a "good" row or not? Normally regex is faster than split. I used a slice to avoid $1. Then there is an "if" and a push if this row is "ok". The regex can become fancier if need be, or sometimes less fancy is ok too...($row =~ m/\s+(\w)\./)[0];