I know what you mean about vague variable names. In this case $hashref_isbn holds everything from our isbn table so for me and colleagues the name does have meaning.

Thanks for your idea about a hash keyed by partial ISBNs! Clever, clever.

Here's what I ended up using:

my @full_isbns = ( defined $hashref_isbn ? keys %{$hashref_isbn} : () 
+);

my %isbn_by_stub = ();
foreach my $full_isbn ( @full_isbns ) {
    my $stub_isbn = substr $full_isbn, 6, 5;
    $isbn_by_stub{$stub_isbn} = $isbn;
    }

# 4-digit ISBN stub
if ( $isbn =~ m/^\d{4}$/ ) {
    $isbn = 0 . $isbn;
    }

# 5-digit ISBN stub
if ( $isbn =~ m/^\d{5}$/ ) {
    if ( exists $isbn_by_stub{$isbn} ) {
        print "Found! $isbn matches $isbn_by_stub{$isbn}\n";
        }
    }
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