This introduces a different error: if the last coin flip is a head, it doesn't get substituted, because it's not followed by a space. So, change the + to a *:

#!/usr/bin/perl

$s.=rand()<.5?H:$"for+a..gr;
$_=$s;
s/(H+) */"|".length$1/ge;
print"$s\n$_|\n$c\n"
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