Just for grins, I took a stab at performance profiling of several solutions. I took out the final print statements. And it could be that my transcription contains some errors. But here are the results that I got with 4 runs: 
#!usr/bin/perl -w
use strict;
use Benchmark;

timethese (1000000,

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

     my @keep = map { my $item = $_; 
                 my $key = ( split )[ 1 ]; 
                 map {$key =~ +/\Q$_/ ? $item : ()} @include 
                } @array;
     },
     
   AnomalousMonk =>
    q{
      my  @array = ('x y.g z 123', 'a b.f c 456', 'd b.c w 321');
      my  @include  = qw(b q);
      my ($include) = map qr{ \b [$_] \b }xms, join '', @include;
      my  $second   = qr{ \b [[:lower:]] \b }xms;
      my  $element  = qr{ $include \. $second }xms;
      my @keep =  grep m{ $element }xms,  @array;
      },
      
    Marshall =>
    q{
       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});
        }    
     },
     
     desemondo =>
     q{
        @array = ("x y.g z 123", "a b.f c 456", "d b.c w 321");
        @include = ("b","q");
        my %testHash = map { $_,1} @include;
     
        foreach my $tmp (@array)
        {
            my @tokens = split /\s+/,$tmp;
            my ($test,$rubbish) = split /\./, $tokens[1];
            push (@keep, $tmp) if (exists $testHash{$test});
        } 
      }
  } 
);

__END__
Benchmark: timing 1000000 iterations of AnomalousMonk, Marshall, 
        desemondo, jwkrahn...
AnomalousMonk:
   27 wallclock secs (26.42 usr +0.16 sys = 26.58 CPU)@37625.10/s 
  Marshall: 
   17 wallclock secs (17.38 usr + 0.00 sys = 17.38 CPU)@57553.96/s 
 desemondo: 
   40 wallclock secs (40.94 usr + 0.11 sys = 41.05 CPU)@24362.32/s 
   jwkrahn: 
   77 wallclock secs (76.48 usr + 0.00 sys = 76.48 CPU)@13074.63/s 

Benchmark: timing 1000000 iterations of AnomalousMonk, Marshall, 
        desemondo, jwkrahn...
AnomalousMonk:
    27 wallclock secs (26.45 usr + 0.09 sys = 26.55 CPU)@37669.04/s
  Marshall: 
     18 wallclock secs (17.36 usr +  0.00 sys = 17.36 CPU)@57607.01/s 
 desemondo: 
    44 wallclock secs (44.05 usr +  0.14 sys = 44.19 CPU)@ 22631.09/s 
   jwkrahn: 
     97 wallclock secs (97.92 usr +  0.02 sys = 97.94 CPU)@10210.65/s 
   
Benchmark: timing 1000000 iterations of AnomalousMonk, Marshall, 
        desemondo, jwkrahn...
AnomalousMonk:
     27 wallclock secs (26.75 usr + 0.11 sys = 26.86 CPU)@37230.08/s 
  Marshall: 
    18 wallclock secs (17.64 usr +  0.00 sys = 17.64 CPU)@56689.34/s 
 desemondo: 
     47 wallclock secs (48.14 usr +  0.09 sys = 48.23 CPU)@20732.26/s 
   jwkrahn: 
     106 wallclock secs (104.81 usr + 0.00 sys = 104.81 CPU)@9540.80/s
   
Benchmark: timing 1000000 iterations of AnomalousMonk, Marshall, 
        desemondo, jwkrahn...
AnomalousMonk:
    28 wallclock secs (27.27 usr + 0.08 sys = 27.34 CPU)@36569.76/s 
  Marshall: 
    21 wallclock secs (20.39 usr +  0.00 sys = 20.39 CPU)@49043.65/s
 desemondo: 
    55 wallclock secs (54.77 usr +  0.11 sys = 54.88 CPU)@18223.23/s 
   jwkrahn: 
    114 wallclock secs (115.05 usr +  0.00 sys = 115.05 CPU)@8692.10/s

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In reply to Re^2: filter an array with consecutive splits by Marshall
in thread filter an array with consecutive splits by coldy

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