The character from the unaffected part of the original value isn't random. It's actually very determinate, as you can see from the below example. 
$s='foobarstuv';
foreach $n (3 .. 5)
{
   for $v (qw[ a ab abc abcd abcde abcdef ])
   {
      $d = substr( $c=$s, 1, $n, $v);
      $f = substr( $e=$s, 1, $n) = $v;
 
      printf "4-arg: '%13s' (%5s) ... 3-arg: '%13s' (%5s)\n", $c, $d, 
+$e, $f;
   }
   print $/;
}

--------------
4-arg: '     faarstuv' (  oob) ... 3-arg: '     faarstuv' (  aar)
4-arg: '    fabarstuv' (  oob) ... 3-arg: '    fabarstuv' (  aba)
4-arg: '   fabcarstuv' (  oob) ... 3-arg: '   fabcarstuv' (  abc)
4-arg: '  fabcdarstuv' (  oob) ... 3-arg: '  fabcdarstuv' (  abc)
4-arg: ' fabcdearstuv' (  oob) ... 3-arg: ' fabcdearstuv' (  abc)
4-arg: 'fabcdefarstuv' (  oob) ... 3-arg: 'fabcdefarstuv' (  abc)
 
4-arg: '      farstuv' ( ooba) ... 3-arg: '      farstuv' ( arst)
4-arg: '     fabrstuv' ( ooba) ... 3-arg: '     fabrstuv' ( abrs)
4-arg: '    fabcrstuv' ( ooba) ... 3-arg: '    fabcrstuv' ( abcr)
4-arg: '   fabcdrstuv' ( ooba) ... 3-arg: '   fabcdrstuv' ( abcd)
4-arg: '  fabcderstuv' ( ooba) ... 3-arg: '  fabcderstuv' ( abcd)
4-arg: ' fabcdefrstuv' ( ooba) ... 3-arg: ' fabcdefrstuv' ( abcd)
 
4-arg: '       fastuv' (oobar) ... 3-arg: '       fastuv' (astuv)
4-arg: '      fabstuv' (oobar) ... 3-arg: '      fabstuv' (abstu)
4-arg: '     fabcstuv' (oobar) ... 3-arg: '     fabcstuv' (abcst)
4-arg: '    fabcdstuv' (oobar) ... 3-arg: '    fabcdstuv' (abcds)
4-arg: '   fabcdestuv' (oobar) ... 3-arg: '   fabcdestuv' (abcde)
4-arg: '  fabcdefstuv' (oobar) ... 3-arg: '  fabcdefstuv' (abcde)

[download]

The extra characters are the characters from the string after substitution, up to the length of the substitution. So, the behavior is quite determinate. As well, it's very consistent. It's just a little counter-intuitive. *shrugs*

------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

In reply to Re2: An Oddity of substr by dragonchild
in thread An Oddity of substr by Zaxo

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