use strict;
use warnings;

my $header = q{};
my $dataAccumulator;

while( <DATA> )
{
    chomp;
    if( m{^>} )
    {
        printDataItem() if $header;
        ( $header ) = m{^.(\S+)};
        $dataAccumulator = q{};
    }
    else
    {
        $dataAccumulator .= $_;
    }
    
}
printDataItem();

sub printDataItem
{
    print
       qq{>$header length=},
       length $dataAccumulator,
       qq{\n$dataAccumulator\n};
}

__DATA__
>IDnumber1 length=350
AGCTG
AAGTCGCT
>IDnumber2 length=350
AGAACGT
ACC
>IDnumber3 length=350
AGC
ACTTCGCTAACT