while (my $inp = <>) {
    $inp =~ s/^\s*(\w+)\s*$/$1/;
    if    ($inp eq 'AB') { AB() }
    elsif ($inp eq 'CD') { CD() }
    ... etc ...    
    else {
        print "Command unknown: '$inp'\n";
    }
}

##</code><code>##

while (my $inp = <>) {
    $inp =~ s/^\s*(\w+)\s*$/$1/;
    given ($inp) {
       when ('AB') { AB() }
       when ('CD') { CD() }
       ... etc ...
       default {
        print "Command unknown: '$inp'\n";
    }
}

##</code><code>##

my %dispatch_table = (
    AB => \&AB,
    CD => \&CD,
    ... etc ...
);
while (my $inp = <>) {
    $inp =~ s/^\s*(\w+)\s*$/$1/;
    if (exists $dispatch_table{$inp}) {
        &{$dispatch_table{$inp}}();
    }
    else {
        print "Command unknown: '$inp'\n";
    }
}