Just for the sake of doing it differently, and because I so love List::Util. (Thanks to merlyn for mentioning it way back here on PM.) :)

use List::Util qw(reduce);
my @num = qw/3.34 3.67 4.75 4.98/;
my @diff = map abs($num[$_-1] - $num[$_]), 1 .. $#num;
my $maxidx = reduce { $diff[$a] > $diff[$b] ? $a : $b } 0 .. $#diff;
print "|$num[$maxidx] - $num[$maxidx+1]| = $diff[$maxidx]\n" if @num >
+ 1;
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Makeshifts last the longest.