$string = '
blah blah blah before-list text
[
line one line 1 line one line 1 
line 2 line two line 2 line two line 
line number 3 line three line number 3 line three 
line 4
]
blah blah later after-list text
';


$string =~ s!
             ^\[$ # open-bracket as entire line  
             (.*) # all including linebreaks - /s modifier
             ^\]$ # close-bracket as entire line  
             !$x=$1;
              $x =~ s/^/<li>/g; # LIs at the start of each line
              "<ol>$x</ol>";    # return whole thing
             !smegx;            # smeg -- how cool is that?

print $string;


##</code><code>##

blah blah blah before-list text
<ol>
<li>line one line 1 line one line 1 
<li>line 2 line two line 2 line two line 
<li>line number 3 line three line number 3 line three 
<li>line 4
</ol>
blah blah later after-list text

##</code><code>##

blah blah blah before-list text
<ol><li>
<li>line one line 1 line one line 1 
<li>line 2 line two line 2 line two line 
<li>line number 3 line three line number 3 line three 
<li>line 4
</ol>
blah blah later after-list text