dynamic variable concatenation

# Taskfile.yml

version: '2'

vars:
  staticTopLevelVar: static
  dynamicTopLevelVar: '{{ .pfx }}:{{ .staticTopLevelVar }}'

.dynamic_vars: &dynamic_vars
  dynamicVar: '{{ .pfx }}:{{ .staticTopLevelVar }}'
  dynamicVarReferencesOtherDynamicVar: '{{ .dynamicVar }}:dynamicVarReferencesOtherDynamicVar'

tasks:
  echo:
    cmds:
      - echo '{{ .staticTopLevelVar }}'
      - echo '{{ .dynamicTopLevelVar }}'
      - echo '{{ .dynamicVar }}'
      - echo '{{ .dynamicVarReferencesOtherDynamicVar }}'
    vars:
      <<: *dynamic_vars
    silent: true

as you can see, you cannot concatenate top level vars.
you have to use task vars to concatenate predefined vars with argument vars.

task echo pfx=my-prefix
static
<no value>:static
my-prefix:static
my-prefix:static:dynamicVarReferencesOtherDynamicVar