Looking at the documentation for Crypt::Simple, it is apparently only the encryption phase which takes a passphrase.  Therefore, it appears you will always get your encrypted data successfully decrypted when using decrypt.

That's not to say the passphrase doesn't matter, though.  You can verify that using a different passphrase produces different results:

#!/usr/bin/perl -w
                                                                      
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use strict;
use warnings;
                                                                      
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use Crypt::Simple qw(encrypt decrypt);
                                                                      
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my $data1 = encrypt("Here we go", passphrase => 'myTestPhrase');
my $data2 = encrypt("Here we go", passphrase => 'AnotherTestPhrase');
print "Encrypted(1): $data1\n";
print "Encrypted(2): $data2\n";
                                                                      
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my $result1 = decrypt($data1);
my $result2 = decrypt($data2);
print "Decrypted(1): $result1\n";
print "Decrypted(2): $result2\n";
[download]

which produces ...

Encrypted(1): Qf9g4cqQ/6xXgqvcPLJA6u9PZ4jPgegPrsBCsFaPch3LDsaN6TjN6xjL
+jebw4yPFNO2rEBTUgISSkiX1v2Aelt4qmHFcd6nF
Encrypted(2): qymmKtF/LXb+HIZe6ZCFS8xtLadYkS+09dZ8zxAvL+r9Rw+GjkeBMc8y
+e++Yl/s/DxJppvKQm2ToJ78mENTuxcc8fUMrKdZbHuWGiMBxgK0=
Decrypted(1): Here we go
Decrypted(2): Here we go
[download]