I don't know much about PHP, but here is a straightforward idea that might make some sense or I hope at least give you something to work from. The general idea is to break this down into smaller "chunks". I assume that the code that your Perl program "re-writes" does compile and that parens match up, etc. Compressing the spaces is also a simplifying assumption.

#!/usr/bin/perl -w
use strict;

my $line = q{  split( '/',   $string )  };
               
$line =~ s/ //g;  #compress spaces
                  #now just "split('/',$string)"

if ($line =~ m/^split/) # check for the split keyword
{
   # get (parm1, parm2) of the split, ie the two
   # things separated by commas within the parens
   my ($parm1,$parm2) = ($line =~ m/\((.*?),(.*?)\)/)[0,1];
   
   # now get stuff between quotes in parm 1
   my $inside_quote = ($parm1 =~ m|'(.*?)'|)[0];
   
   # change any / to \/
   $inside_quote =~ s|/|\\/|g;
   
   #now just print back out
   print "preg_split('/$inside_quote/',$parm2)\n";
}

__END__
Prints:
preg_split('/\//',$string)
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