Here's an example with use vars. Note that, although the use vars is within a block, it takes effect for the whole file. However, after the switch to package main, $var refers to $main::var.

#!perl -l

package Pkg;

{
  use vars qw/ $var /;

  $var = 7;

  print $var;          # 7
}

print $var;            # 7

package main;

print $var;            # undef; $main::var has not been set
                       # error with strict

print $Pkg::var;       # 7
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Here's an example with my. Note that only within the lexical block can you access the $var that holds the number 7.

#!perl -l

package Pkg;

{
  my $var = 7;

  print $var;          # 7
}

print $var;            # undef; $Pkg::var has not been set
                       # error with strict

package main;

print $var;            # undef; $main::var has not been set
                       # error with strict

print $Pkg::var;       # undef; $Pkg::var has not been set
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Here's an example with our. Note the behavior outside the block before the switch to main; $var refers to the $Pkg::var package variable, but without the package qualifier it would be an error with use strict.

#!perl -l

package Pkg;

{
  our $var = 7;

  print $var;          # 7
}

print $var;            # 7
                       # error with strict

package main;

print $var;            # undef; $main::var has not been set
                       # error with strict

print $Pkg::var;       # 7
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Here's another example with our. Note that, after the switch to package main, $var still refers to $Pkg::var, because we're still in the same lexical scope.

#!perl -l

package Pkg;

our $var = 7;

print $var;            # 7

package main;

print $var;            # 7; even though we switched packages,
                       # $var still refers to $Pkg::var
                       # in this lexical scope

print $Pkg::var;       # 7
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So, both use vars and our declare a variable in the current package. The difference between them is in how you can access the variable. With either one, you can always use $Pkg::var from anywhere. With use vars, you can use $var anywhere in the file, as long as you're in the right package. With our and use strict, you can only use $var within the lexical scope, but you can still use $var after switching packages.

Finally, to answer your question about which of three declarations to use at the top of a script. If you're only accessing the variables from that file, you can use my. If you don't switch to a different package later on in the file, use vars and our are equivalent. If you do switch packages, our could result in surprising behavior, as in the last example above.

 

Update: echo pointed out a few places where I had written $variable instead of $var.