package Blah; {
    sub foo {
        print "bar!\n";
    }
}

package main; {
    foo();
}
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This code will define  foo() in the  Blah package; it will not be available in the  main package unless exported to it or invoked as a fully qualified subroutine (called without warnings or strictures):

c:\@Work\Perl\monks\R0b0t1>perl -le
"package Blah; {
     sub foo {
         print \"bar!\n\";
     }
 }
 package main; {
     foo();
 }
"
Undefined subroutine &main::foo called at -e line 1.

c:\@Work\Perl\monks\R0b0t1>perl -le
"package Blah; {
     sub foo {
         print \"bar!\n\";
     }
 }
 package main; {
     Blah::foo();
 }
"
bar!
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On a tangential note, the package syntax

{ package Foo;
  my $x = ...;
  ...
  sub bar { ... }
  ...
  }
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and from Perl version 5.14 onward

package Foo {
  my $x = ...;
  ...
  sub bar { ... }
  ...
  }
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will cause the asserted package to be "turned off" at the end of the block, with reversion to the "original" package. E.g., (with full warnings and strictures):

c:\@Work\Perl\monks\R0b0t1>perl -wMstrict -le
"print 'perl version: ', $];
 ;;
 in_pkg('A');
 ;;
 package Foo {
   ::in_pkg('B');
   }
 ;;
 in_pkg('C');
 ;;
 sub in_pkg { print qq{$_[0]: in package }, scalar caller; }
"
perl version: 5.014004
A: in package main
B: in package Foo
C: in package main
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