HI, im having trouble with this task 
Write a perl script that will generate a new output file (“task1 output.txt”) which contains
the sequence name, length, and GC-content for each sequence. There should be a header
line which identifies the contents of columns (so the first line in the output file should be
“SeqName Length GC-Content” or something similar). The GC-content of a sequence is
defined as the percentage of bases that are G or C (from 0% to 100%), and a high GCcontent
is associated with coding sequences.

>Seq1
ACGT
Then your output file should look like:
SeqName Length GC-Content
Seq1              4           50

I can do the in and out for the file handles, but im confused as to what to put in my while loop. and how will it know to match in the file ?
Like mentioned, GC content is the percentage of bases that are G or C in the sequences. Percentage is calculated quite easily using basic math once you obtain the counts of bases that are G or C in and the total sequence length for each sequence. Your loop with iterate through the file, executing its code block for each sequence it finds.

I cannot - and I hope others do not too - provide code. That would cripple learning.