If you used defined $USER{coffee} as a check it will actually autovivify (automatically create) the coffee key as it does when you do your current test.

That's not true:

use warnings;
use strict;

my %c = ( hey=>'Joe',
      where=>'you goin\'',
      with=>'that gun in your hand');

if (defined $c{Hendrix}) {
    print "Jimi is alive and well\n";
}

foreach ( sort keys %c ) {
    print "Key : $_ => $c{$_}\n";
}
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This does not print "Hendrix" and yield up a warning on the loop that goes through the keys (Perl 5.6.1, Win32).

Of course defined $hash{key} and exists $hash{key} do test for different things:

$hash{key} = undef; #defined $hash{key} false, exists $hash{key} true
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perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
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