I'm guessing some pattern matching and if/elsif/else blocks are what you need

!perl
use strict;
my %groups = ();
while (my $code = <DATA>){
  chomp($code);
  my $group = code2group($code);
  print "Code $code is Group $group\n";
}

sub code2group {
  my $code = shift;
  if ($code eq '277'){
    return 'B';
  } elsif ($code =~ /2770[012379]?/){
    return 'A';
  } else {
    return 'UNKNOWN';  
  }
}

__DATA__
277
2770
27700
27701
27702
27703
27707
27709
0704 
07049
12345
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