my %word;

open my $fh, "<", "/usr/dict/words";
chomp, push @{$word{substr $_, 0, 1}}, $_ while <$fh>;
close $fh;

##</code><code>##

if(grep $_ eq $user_input, @{$word{substr $user_input, 0, 1} || []}) {
    # it's there
}
else {
    # no, that wasn't it
}

##</code><code>##

chomp, $_ = lc, push @{$word{substr($_, 0, 1)}}, $_ while <$fh>;

# and

grep lc eq $user_input, @{$word{substr($user_input, 0, 1)} || []}