#!/usr/bin/perl

# Demonstrate different loops for use with string substitutions

use strict;
use warnings;
# use diagnostics;

my $NewRev = "\tabc\tkis17.1.33en_12345.exe<h href";
my $RedirectUrl;

s/.*kis17/17/, s/.exe//, s/en_/./ for $NewRev; #you assigned null to $NewRev
print "for at the end: \$NewRev = <$NewRev>\n\n";

$NewRev = "\tabc\tkis17.1.33en_12345.exe<h href"; #set back to original
$RedirectUrl = $NewRev =~ s/.*kis17/17/r =~ s/.exe//r =~ s/en_/./r ; 
print "run on line: \$RedirectUrl = <$RedirectUrl>\n"; #modified
print "run on line: \$NewRev = <$NewRev>\n\n";         #not modified

$NewRev = "\tabc\tkis17.1.33en_12345.exe<h href"; #set back to original
for ($NewRev) {s/.*kis17/17/; s/.exe//; s/en_//; }
print "for at the start: \$NewRev = <$NewRev>\n\n";

$NewRev = "\tabc\tkis17.1.33en_12345.exe<h href"; #set back to original
for ($NewRev)  #white spaces consumes no Mips! why so compact?
{
   s/.*kis17/17/;   #all 3 statements modify $NewRev
   s/.exe//; 
   s/en_//; 
}
print "for at the start: \$NewRev = <$NewRev>\n\n";


__END__

Need __END__ not just END
But of course __END__ is optional, just use it like this when you
want to put something in the code like an example printout

Prints:
for at the end: $NewRev = <17.1.33.12345<h href>

run on line: $RedirectUrl = <17.1.33.12345<h href>
run on line: $NewRev = <	abc	kis17.1.33en_12345.exe<h href>

for at the start: $NewRev = <17.1.3312345<h href>

for at the start: $NewRev = <17.1.3312345<h href>