From perldoc:substr:

substr - get or alter a portion of a string 

SYNOPSIS:
=========
substr EXPR,OFFSET,LEN,REPLACEMENT 
substr EXPR,OFFSET,LEN 
substr EXPR,OFFSET 

DESCRIPTION:
============
Extracts a substring out of EXPR and returns it. First
character is at offset 0, or whatever you've set $[ to
(but don't do that). If OFFSET is negative (or more
precisely, less than $[ ), starts that far from the end of
the string. If LEN is omitted, returns everything to the
end of the string. If LEN is negative, leaves that many
characters off the end of the string.

So, your -1 where the OFFSET is at, is starting from the end of the string. And since you
are giving the length, you are getting only the last character of your string.

UPDATE: You might also want to read this

TStanley
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Never underestimate the power of very stupid people in large groups -- Anonymous