my $s = do {local $/=undef; <DATA>};
while( $s =~ s#<span style="(font-weight: bold;|font-style: italic;|text-decoration: underline;)">(?!.*?<span)(.*?)</span>#span2tag($1,$2)#sgei ){};
print $s;

sub span2tag {
  my ($attr, $s) = @_;
  return "<b>$s</b>" if $attr =~ /bold/;
  return "<i>$s</i>" if $attr =~ /italic/;
  return "<u>$s</u>" if $attr =~ /underline/;
  return $s;
}
__DATA__
this <span style="font-weight: bold;">is</span> some
<span style="font-weight: bold;">test
  <span style="font-style: italic;">text</span>
  <span style="text-decoration: underline;">for</span> bolding</span>, underlining and italicizing text.<br />