One way I see how it could be done is passing the Minimum Energy filename to the script, here in untested code, assuming you do not store the output results from the minimum enery:

./leg.pl `mineng.pl |grep "This was in file"|awk '{print $5}'`
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And then in leg.pl:

my $minfilename = shift; # = $ARGV[0];
:
:
#somewhere in the loop:
if($f eq $minfilename){
 #store values to substract them later
}
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However, if you DO save that file and have a known filename, lets call it min.txt, then this also works:

my $minfilename = `grep "This was in file" min.txt|awk '{print $5}'`;
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And go from there, testing against $f to get its values.