I have tried your code with two different variables as below, Is works as expected.

use strict;
use warnings;

my $a = 10;
my $b = 10;

#IMHO: $a-- should finish the current operation and then -1
if ($a == $b--)
{
    print "\n T1";
    
}

$b=10;
#IMHO: --$a should -1 first and continute with current operation , i.e
+ if compare
if ($a == --$b)
{
    print "\n T2";
}

#ENV: #Active Perl 5.20, Windows 7, x64 

#Output
# T2
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Unary

The auto-decrement (--), and auto-increment (++) operators are unary operators. They alter the scalar variable they operate on by one logical unit. On numbers, they add or subtract one.Operators that come in post- and pre- varieties can be used two ways. The first way returns the value of the variable before it was altered, and the second way returns the value of the variable after it was altered.

my $foo = 1;
 
# post decrement (printed and then decremented to 0)
print $foo--; # prints 1
print $foo;   # prints 0
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my $foo = 1;
 
# pre-decrement (decremented to 0 then printed)
print --$foo;  # prints 0
print $foo;    # prints 0
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