As you say the use warnings flag would have told you you were calling my on a variable already declared in the same scope. compare your code with that below

#!/usr/local/bin/perl
use strict;

my $foo = 1;
my $fum = 1;
print STDERR "variables are now: $foo,$fum\n";
&foo();
{
my $foo = 2;
   $fum = 2;
   print STDERR "variables are now: $foo,$fum\n";
}
sub foo {
       print STDERR "variables are now: $foo,$fum\n";
}

variables are now: 1,1
variables are now: 1,1
variables are now: 2,2
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By enclosing the redeclaration in a block you avoid scope confusion