use strict;
use warnings;

my ($head);

while(<DATA>){
     my ($x, $str, $len, $s);
     $s = $_;

     if($s=~m/^>/){
        chomp($s);
        ($head)=(split(/ /,$s))[0];
     }
     else{ 
        chomp($s);
        $len=length($s);
     }
     print "$head length=$len\n$s\n" if($s !~m/^>/);
}

__DATA__
>IDnumber1 length=350
AGCTG
>IDnumber2 length=350
AGAACGT
>IDnumber3 length=350
AGC


output:
-------
>IDnumber1 length=5
AGCTG
>IDnumber2 length=7
AGAACGT
>IDnumber3 length=3
AGC