I tried to reproduce but without luck, it complains independently of strict:

#!/usr/bin/perl
#use strict;
#use warnings;
#use diagnostics;

my $a;
my @array = sort { $a <=> $b } 19, 28, 1 .. 3;
print "@array\n";
__END__
Can't use "my $a" in sort comparison at oha2.pl line 7
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Which version of perl are you using? I tested it with 5.8.4 and 5.8.8, both on Linux.