s@$var@$somethingelse@

I presume you want the replacement text to be the literal string '$somethingelse', rather than the contents of the variable. The replacement text in substitutions behave like double-quoted strings, which cause interpolation. To avoid this, either use single quotes, or escape the $, eg

s{$var}'$somethingelse';
s@$var@\$somethingelse@;
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Dave.