Note that if you need to disambiguate between two strings that match for the first few characters you can require some number of characters thus:

Or else one may proceed like thus:

my @matching=grep { 0 == index $_, $userinput } @commands;
[download]

Then take actions according to whether @matching==0, @matching==1 or @matching>=2. In the latter case depending on what the commands are actually supposed to do I would either emit a warning and decide upon priority (which may be given by the ordering of @commands itself) or ask the users to refine their input.

Update: minimal example follows.

#!/usr/bin/perl

use strict;
use warnings;

my @commands=qw/foo bar baz/;
print "Choose command [@commands]\n";
{
    chomp(my $cmd=<STDIN>);
    my @ok=grep { 0 == index $_, $cmd } @commands;
    print "You chose [@ok]\n" and last if @ok==1;
    warn +(@ok>1 ?
        "Ambiguos command [@ok]" :
        "Invalid command"), ", retry!\n";
    redo;
}

__END__
[download]

Usage example:

$ ./foo.pl
Choose command [foo bar baz]
fred
Invalid command, retry!
fo
You chose [foo]
$ ./foo.pl
Choose command [foo bar baz]
ba
Ambiguos command [bar baz], retry!
bar
You chose [bar]
[download]