input file:
----------
>IDnumber1 length=350
AGCTG
>IDnumber2 length=350
AGAACGT
>IDnumber3 length=350
AGC

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

code:
-----
#!/usr/bin/perl
open FH,"inputfile";
while(<FH>){
     $head=();$str=();
     if($_=~m/^>/){
        ($head,$x)=split(' ',$_);
     }
     else{
        $str=$_;
        chomp($str);
        $len=length($str);
     }
     print "$head Length=$len\n";
     print "$str\n";
}