use strict;
my $str = '<b> текст <i>вложение <br> </i> переход</b><b> текст <i>вложение <br> </i> переход</b><b> текст <i>вложение <br> </i> переход</b>';
my $regex = qr/<(b|i)>(.*?)(<br>)+(.*?)<\/\1>/p;
my $subst = '<$1>$2</$1>$3<$1>$4</$1>';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html