Here's a graphic illustration of the use of the "e" for "execute" modifier used on the end of substitute operation in Perl.

The "s" for substitute allows you to replace a matched pattern with a STRING in which you can use special references like \1 or $1 for the first matched substring. If you want it to replace the matched pattern with the RESULT OF RUNNING CODE, you should add that extra e on the end.

In this example, the $emma variable ends up containing a corrupted email address which actually includes some code, whereas (with the "e"), the $emily variable results in a correct email address with the section before the "@" pushed to lower case.

$emma = 'Graham@wellho.net';
$emily = 'Graham@wellho.net';
 
$emma =~ s/(\w+)@/lc($1)."@"/;
$emily =~ s/(\w+)@/lc($1)."@"/e;
 
print ("$emma\n$emily\n");
 
__END__
 
Dorothy:plpw grahamellis$ perl swithe
lc(Graham)."@"wellho.net
graham@wellho.net
Dorothy:plpw grahamellis$
(written 2008-12-16 07:58:51)

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