Your terminology is very confusing, but it sounds to me like you want the numerical value 64₁₆ aka 100₁₀ from the string "64". For that, you can use hex.

$ perl -E'say 0x64 == hex("64") ?"yes":"no"'
yes
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In case I misunderstood, here are some other related conversions:

numerical value 6416 from string "64"	hex("64")
numerical value 6416 from string "100"	"100"
packed byte 6416 from numerical value 6416	chr(0x64)
packed byte 6416 from string "64"	chr(hex("64"))
packed byte 6416 from string "100"	chr("100")
string "64" from numerical value 6416	sprintf("%X", 0x64)

pack 'C' would work in lieu of chr.

Update: I just noticed your followup. It seems that you want the packed byte 64₁₆ (aka "d" in ASCII), but you didn't specify from what.

$ perl -E'say "\x64" eq chr(hex("64")) ?"yes":"no"'
yes

$ perl -E'say "\x64" eq chr(0x64) ?"yes":"no"'
yes
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That could also be written

$ perl -E'say "\x64" eq pack("C", hex("64")) ?"yes":"no"'
yes

$ perl -E'say "\x64" eq pack("C", 0x64) ?"yes":"no"'
yes
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