One possible approach:

>perl -wMstrict -le
"my $bad =
   'x >lt;amp; y >amp;lt; z &>< y lt;gt; z';
 print qq{'$bad'};
 ;;
 my $tity = qr{ (?: amp | gt | lt) ; }xms;
 ;;
 (my $fixed = $bad) =~
   s{ (?: (?: \G (?<! \A)) | &) $tity \K (?= $tity)  }
    '&'xmsg;
 print qq{'$fixed'};
"
'x &amp;gt;lt;amp; y &gt;amp;lt; z &amp;&gt;&lt; y lt;gt; z'
'x &amp;&gt;&lt;&amp; y &gt;&amp;&lt; z &amp;&gt;&lt; y lt;gt; z'
[download]

#!/usr/bin/perl
use 5.016;
use strict;
use warnings;

# 1040996

my @strs = ("foo bar >lt;amp; blivitz", 
            "FOO BAR BLIVITZ, >apos; "sect; ",
            "no entities here",
            "But there are ¢pound; entities for 'cent' and 'pound
+' here.",
            );

for my $str(@strs) {
    if ( $str =~ /(&[^ ]+)/ ) {   # match any ampersand followed by on
+e or 
                                  # more NON-spaces (aka \S; see below
+)
        my $found = $1;
        say "DEBUG: found semicolon(s) at |$found| in \"$str\"";
        if ($str =~ /
                [^&;]*?     # anything that's neither "&" nor ";"
                (&.+)       # followed by an ampersand and multiple ch
+ars
                (?!\S)      # until prev capture is followed by someth
+ing 
                            # NOT-a-space ("negative lookahead")
                /gx )    {  # globally, extended notation, end conditi
+ons, begin actions
            my $substr = $1;
            say "\$substr: $substr\n";
            (my $fixed = $substr ) =~ s/(;)([a-z])/$1&$2/g;
            say "\$fixed: $fixed \n";
        }
    } else {
        say "\n\t No html entities found.\n";
    }
}

=head

C:\>1040996.pl
DEBUG: found semicolon(s) at |>lt;amp;| in "foo bar >lt;
+amp; blivitz"
$substr: >lt;amp; blivitz

$fixed: &><& blivitz

DEBUG: found semicolon(s) at |>apos;| in "FOO BAR BLIVITZ, >apos
+; "sect; "
$substr: >apos; "sect;

$fixed: >' "§


         No html entities found.

DEBUG: found semicolon(s) at |¢pound;| in "But there are ¢po
+und; entities for 'cent' and 'pound' here."
$substr: ¢pound; entities for 'cent' and 'pound' here.

$fixed: ¢£ entities for 'cent' and 'pound' here.


=cut
[download]

BUT... this is imperfect (note the captured, trailing "blivitz" in the first example, and the loss of non-entity material at the beginning of each array element. That's easily enough fixed with a little more code.

ANOTHER BUT: This breaks on some edge cases... such as a line where the bad entities immediately precede the EOL.

Update: added word "general" in first graf.

 perl -e '$x=qw(&apm;gt;lt;amp;);$x=~s/;(.)/;&$1/g;print $x,"\n";'
[download]

Thanks, but this is just a solution for this particular example; I need a general solution for any codes.

You need to implement a lookup table that contains a list of all possible, or otherwise a subset, of HTML character codes.

Therefore, each time your program encounters

code;

instead of

&code;

, it will need to add an ampersand at the start.

I am not sure my proposal will not create havoc elsewhere in your html (ie how robust it is) but have a look:

use strict;
use warnings;
my $html = ">lt;amp; something else <gt;amp;something else ;
+";
$html =~ s|&(([^ ;]+;)+)| join '', map { "&$_;" } split /;/, $1 |ge;
print "$html\n";
[download]

UPDATE: Probably \w is more robust than [^ ;].

You can replace the entities even if they are already right with s{ \&? \b (amp|lt|gt) ; }{&$1;}xg;. And you can generate the OR list (in parenthesis) with a map. Edit : Here the &? catches the ampersand if it's there so that the substitution does not add another one. The \b makes sure that you have something that looks like a failed HTML entity. You wouldn't want "Tom bought a lamp; he loves lamps" to become "Tom bought a l& he loves lamps" for example.

$_ = "test;a  <>amp;kap;&da;ma; &amp alfa&romeo & mich;";

while ( s/([&][^ ]*);(?!( |&|$))/\1AMPSEMICOLON/ ) {};
s/AMPSEMICOLON/;\&/g;

# Output:
test;a  &<>&&kap;&da;&ma; &amp alfa&romeo & mich;
[download]

... not restricted to a particular list of html codes ...

I'm not sure that's really such a good idea (it should be easy to get a list of all HTML entities you could possibly be interested in), but here's a generalization:

>perl -wMstrict -le
"my $bad =
   'x &foo;xx;yz;qwe; y &de;fghj;h; z &>< y lt;gt; z';
 print qq{'$bad'};
 ;;
 my $tity = qr{ [[:alpha:]]+ ; }xms;
 ;;
 (my $fixed = $bad) =~
   s{ (?: (?: \G (?<! \A)) | &) $tity \K (?= $tity) }
    '&'xmsg;
 print qq{'$fixed'};
"
'x &foo;xx;yz;qwe; y &de;fghj;h; z &amp;&gt;&lt; y lt;gt; z'
'x &foo;&xx;&yz;&qwe; y &de;&fghj;&h; z &amp;&gt;&lt; y lt;gt; z'
[download]