I much prefer the 'unless' version of this as for me (who needs new glasses) I can sometimes miss the ! so I prefer.

unless(open( FH, "<$file") ) {
    # Error code, logging, bits, bobs (etc etc)
    die "Cannot open $file :$!\n";
}
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As a matter of personal taste I think this is more natural (to me).. unless this do this ;P