You can use Text::Abbrev, which is a core module:

use Text::Abbrev;
use strict;

print "Enter a command: ";
chomp(my $action = <STDIN>);  # they could enter 'adm' or 'h'
my %modes = abbrev(qw( hsbn admin ... ));

# if $modes{$action} is defined, that means it is a
# NON-AMBIGUOUS abbreviation of 'hsbn', 'admin', etc.
if (defined(my $fullaction = $modes{$action})) {
  if ($fullaction eq 'hsbn') { ... }
  elsif ($fullaction eq 'admin') { ... }
  ...
}
else {
  print "$action was ambiguous.\n";
}
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You'll have to do more work if you want to print a list of commands that matched the ambiguous action they've entered. In which case you'll probably just want to avoid Text::Abbrev altogether and roll your own solution.

Which reminds me: I think I saw this in the Perl Cookbook... if you reverse the arguments of the pattern match, you can get the desired effect:

if ("hsbn" =~ /^$action/) {
  # $action is "h", "hs", "hsb", or "hsbn"
}
[download]

Update (8:30 AM EDT): I just figured out a nifty way to do abbreviation matching and ambiguity reporting at the same time, with a regex, of course.

my $valid = join "\n", qw( hsbn admin help quit ... );
chomp(my $action = <STDIN>);
my @matches = $valid =~ /^($action.*)/mg;

if (@matches == 0) {
  print "No command matched '$action'\n";
}
elsif (@matches > 1) {
  print "Multiple commands matched '$action': [@matches]\n";
}
else {
  print "You selected $matches[0].\n";
}
[download]

I think it's pretty clever. $valid is a string with newlines between every complete action name: "hsbn\nadmin\nhelp\nquit". The regex /^($action.*)/mg does a few things. First, the /m modifier means that the ^ in the regex matches at the beginning of the string OR after any internal newline. The ($action.*) part matches not only the abbreviation the user entered, but also the rest of the non-newline characters after it (i.e. the completion of the action name). The /g modifier returns all matches found, so that if they only entered 'h', you'd get back ("hsbn", "help"). I hope that's helpful for you.

UPDATE (9:30 PM EDT): Text::Abbreviate, coming to a CPAN mirror near you...