Your code won't work for several reasons: $file is a scalar and in the context of your foreach-loop it is a list of one element. $number will be a lexical alias for $file while the single iteration of the loop happens. $number_counter will be increased once and $difference will be assigned 1, for a-(a-1)==a-a+1==1 .

Try this:

my @number = qw(3.34 3.67 4.75 4.98);

my $maxdiff = 0;
my @index = ();

foreach my $i ( 1 .. @number-1 ){
    my $diff = abs( $number[$i] - $number[$i-1] );
    if( $diff > $maxdiff ){
        $maxdiff = $diff;
        @index = ($i-1,$i);
    }
}

if( @index ){
    printf "%f: %f , %f\n",
        $maxdiff, @number[ @index ]
}
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update: of course, see perlsyn and perldata

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