use Internals qw[ SetReadOnly ];

my %hash; 
undef @hash{ qw[ the quick brown fox ] };
SetReadOnly( \%hash );
...

while( <$fh> ) {
    if( ... some capturing regex ... ) { 
        $hash{ $1 } = $2;
    }
    elsif( ... regex ... ) {
        $hash{ $1 } = $2;
    }
    else {
        $hash{ fox } == FOX_DEFAULT;
    }
    last if keys %hash == grep defined, values( %hash ); ## Updated, see below.
}