use strict;

my $html = do { local $/; <DATA> };

$html =~ s/(<[^>]+>\s*)(.*)(?=\s*<[^>]+>)/$1<b>$2<\/b>/gm;

print "$html\n";

__DATA__
<p>
blah blah
<blockquote>
blah
</blockquote>
blah
<p>
[download]

And the output -

<p>
<b>blah blah</b>
<blockquote>
<b>blah</b>
</blockquote>
<b>blah</b>
<p>
[download]