if ("\U$listaccountlocked[1]" eq 'TRUE') {
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is just an obfuscated way of writing

if (uc($listaccountlocked[1]) eq 'TRUE') {
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From a performance point of view, the former is a proper subset of the latter. Not only does \U calls uc(), it creates an extra copy of the string.

$ perl -MO=Concise,-exec -e'my $y = "\U$x";'
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v:{
3  <$> gvsv(*x) s
4  <1> uc[t2] sK/1
5  <@> stringify[t3] sK/1         <--- This addition is the only
6  <0> padsv[$y:1,2] sRM*/LVINTRO      difference. It creates a
7  <2> sassign vKS/2                   copy of the string.
8  <@> leave[1 ref] vKP/REFC
-e syntax OK

$ perl -MO=Concise,-exec -e'my $y = uc($x);'
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v:{
3  <$> gvsv(*x) s
4  <1> uc[t2] sK/1
5  <0> padsv[$y:1,2] sRM*/LVINTRO
6  <2> sassign vKS/2
7  <@> leave[1 ref] vKP/REFC
-e syntax OK
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