I need some help witch this Perl programing question

Break down the following problem into sub-problems. Indicate input and output data. The solution has to include at least two sub-problems in addition to input and output steps. The sub-problems should be clearly indicated and thoroughly described.

In a file containing rows with DNA sequences, output those not having a start codon, i.e., those strings that do not contain the pattern 'ATG'.

Example file:

ATTTTATGAAAAATTT

ATTTTATGAACCATTT

ATTACATGAAAAATTT