The goal of this exercise is to practice string handling. You
basically need to use functions index, substr and split. In the second
phase, its sufficient to use the sort function.

Task: 

Produce a sorted set of virt. - occ. orbital energy differences, given
following input:

 The electronic state is 1-SGG.
 Alpha  occ. eigenvalues --   -0.27448  -0.27448  -0.26801  -0.26801  -0.26335
 Alpha  occ. eigenvalues --   -0.25992  -0.25992  -0.25881  -0.25881  -0.25630
 Alpha  occ. eigenvalues --   -0.25257  -0.25257  -0.25129  -0.24488  -0.24084
 Alpha  occ. eigenvalues --   -0.23960  -0.23960
 Alpha virt. eigenvalues --   -0.22118  -0.16758  -0.09795  -0.06680  -0.06680
 Alpha virt. eigenvalues --   -0.05189  -0.05189  -0.04384  -0.02944  -0.02944
 Alpha virt. eigenvalues --    0.00231   0.00231   0.01770   0.03559   0.03611
 Alpha virt. eigenvalues --    0.03742   0.03742   0.06332   0.07490   0.07490
 Alpha virt. eigenvalues --    0.07566   0.08138   0.08138   0.09503   0.09503
 Alpha virt. eigenvalues --    0.10922   0.11238   0.11238   0.13825   0.13825
 Alpha virt. eigenvalues --    0.14176   0.16297   0.16297   0.17893   0.18998

Basically, go through *all* combinations of (occ, virt) energy pairs,
form the list of differences, and sort it. The first item should be:

0.23960-0.22118= .01842

Pay attention that sort @list uses text comparison. use sort { $a <=>
$b} for the numeric one.