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

##</code><code>##

#!/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__

##</code><code>##

$ ./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]