You may have code like this:

my $txt = 'hello world';
print "$txt\n";           # prints "hello world"
my $txt = 'oh noes';
print "$txt\n";           # prints "oh noes"
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Instead, you may want this:

my $txt = 'hello world';
print "$txt\n";
$txt = 'oh noes';         # note, no "my"
print "$txt\n";
[download]

It does the same thing, but it doesn't produce a warning. If you use diagnostics, it will tell you this about it:

(W misc) A "my" or "our" variable has been redeclared in the current scope or statement, effectively eliminating all access to the previous instance. This is almost always a typographical error. Note that the earlier variable will still exist until the end of the scope or until all closure referents to it are destroyed.

my $var;
$var=1;
my $var;
$var=2;
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The second $var "masks earlier declaration in same scope". You declare the same variable name a second time.

Has anyone encountered this before? If so, where am I going wrong?
Easily answered using a search engine, like this or this
Since you're new, you should add
use diagnostic;
to your program, so you'll get more detailed explanation, like "%s" variable %s masks earlier declaration in same %s